support for user directories, based on uhexen2 and tyrquake:
** against quakespasm svn revision 588.
** on-the-fly game directory switching not supported yet.
** minimally tested, needs more work.

Index: Quake/sys_sdl_unix.c
===================================================================
--- Quake/sys_sdl_unix.c	(revision 588)
+++ Quake/sys_sdl_unix.c	(working copy)
@@ -20,6 +20,8 @@
 
 */
 
+#define USE_PASSWORD_FILE 1
+
 #include "quakedef.h"
 
 #include <sys/types.h>
@@ -29,6 +31,9 @@
 #include <sys/time.h>
 #include <fcntl.h>
 #include <time.h>
+#if USE_PASSWORD_FILE
+#include <pwd.h>
+#endif	/* USE_PASSWORD_FILE */
 
 #include "SDL.h"
 
@@ -139,9 +144,43 @@
 	return -1;
 }
 
+#define SYS_USERDIR	".quakespasm"
+static char	userdir[MAX_OSPATH];
+
+static void Sys_GetUserdir (char *dst, size_t dstsize)
+{
+	size_t		n;
+	const char	*home_dir = NULL;
+#if USE_PASSWORD_FILE
+	struct passwd	*pwent;
+
+	pwent = getpwuid( getuid() );
+	if (pwent == NULL)
+		perror("getpwuid");
+	else
+		home_dir = pwent->pw_dir;
+#endif
+	if (home_dir == NULL)
+		home_dir = getenv("HOME");
+	if (home_dir == NULL)
+		Sys_Error ("Couldn't determine userspace directory");
+
+/* what would be a maximum path for a file in the user's directory...
+ * $HOME/SYS_USERDIR/game_dir/dirname1/dirname2/dirname3/filename.ext
+ * still fits in the MAX_OSPATH == 256 definition, but just in case :
+ */
+	n = strlen(home_dir) + strlen(SYS_USERDIR) + 50;
+	if (n >= dstsize)
+		Sys_Error ("Insufficient array size for userspace directory");
+
+	q_snprintf (dst, dstsize, "%s/%s", home_dir, SYS_USERDIR);
+}
+
 void Sys_Init (void)
 {
-	host_parms->userdir = host_parms->basedir; /* TODO: implement properly! */
+	Sys_GetUserdir(userdir, sizeof(userdir));
+	Sys_mkdir (userdir);
+	host_parms->userdir = userdir;
 }
 
 void Sys_mkdir (const char *path)
Index: Quake/common.c
===================================================================
--- Quake/common.c	(revision 588)
+++ Quake/common.c	(working copy)
@@ -1846,32 +1846,34 @@ pack_t *COM_LoadPackFile (const char *pa
 COM_AddGameDirectory -- johnfitz -- modified based on topaz's tutorial
 =================
 */
-static void COM_AddGameDirectory (const char *dir)
+static void COM_AddGameDirectory (const char *base, const char *dir)
 {
 	int i;
 	unsigned int path_id;
 	searchpath_t *search;
 	pack_t *pak;
 	char pakfile[MAX_OSPATH];
+	qboolean been_here = false;
 
-	q_strlcpy (com_gamedir, dir, sizeof(com_gamedir));
+	q_strlcpy (com_gamedir, va("%s/%s", base, dir), sizeof(com_gamedir));
 
 	// assign a path_id to this game directory
 	if (com_searchpaths)
 		path_id = com_searchpaths->path_id << 1;
 	else	path_id = 1U;
 
+_add_path:
 	// add the directory to the search path
 	search = (searchpath_t *) Z_Malloc(sizeof(searchpath_t));
 	search->path_id = path_id;
-	q_strlcpy (search->filename, dir, sizeof(search->filename));
+	q_strlcpy (search->filename, com_gamedir, sizeof(search->filename));
 	search->next = com_searchpaths;
 	com_searchpaths = search;
 
 	// add any pak files in the format pak0.pak pak1.pak, ...
 	for (i = 0; ; i++)
 	{
-		q_snprintf (pakfile, sizeof(pakfile), "%s/pak%i.pak", dir, i);
+		q_snprintf (pakfile, sizeof(pakfile), "%s/pak%i.pak", com_gamedir, i);
 		pak = COM_LoadPackFile (pakfile);
 		if (!pak)
 			break;
@@ -1881,6 +1883,14 @@ static void COM_AddGameDirectory (const 
 		search->next = com_searchpaths;
 		com_searchpaths = search;
 	}
+
+	if (!been_here && host_parms->userdir != host_parms->basedir)
+	{
+		been_here = true;
+		q_strlcpy(com_gamedir, va("%s/%s", host_parms->userdir, dir), sizeof(com_gamedir));
+		Sys_mkdir(com_gamedir);
+		goto _add_path;
+	}
 }
 
 #if defined(USE_QS_CONBACK)
@@ -1935,8 +1945,7 @@ void COM_InitFilesystem (void) //johnfit
 	}
 
 	// start up with GAMENAME by default (id1)
-	COM_AddGameDirectory (va("%s/"GAMENAME, com_basedir));
-	q_strlcpy (com_gamedir, va("%s/"GAMENAME, com_basedir), sizeof(com_gamedir));
+	COM_AddGameDirectory (com_basedir, GAMENAME);
 
 #if defined(USE_QS_CONBACK)
 	if (!fitzmode)
@@ -1950,17 +1959,17 @@ void COM_InitFilesystem (void) //johnfit
 	com_nummissionpacks = 0;
 	if (COM_CheckParm ("-rogue"))
 	{
-		COM_AddGameDirectory (va("%s/rogue", com_basedir));
+		COM_AddGameDirectory (com_basedir, "rogue");
 		com_nummissionpacks++;
 	}
 	if (COM_CheckParm ("-hipnotic"))
 	{
-		COM_AddGameDirectory (va("%s/hipnotic", com_basedir));
+		COM_AddGameDirectory (com_basedir, "hipnotic");
 		com_nummissionpacks++;
 	}
 	if (COM_CheckParm ("-quoth"))
 	{
-		COM_AddGameDirectory (va("%s/quoth", com_basedir));
+		COM_AddGameDirectory (com_basedir, "quoth");
 		com_nummissionpacks++;
 	}
 	//johnfitz
@@ -1969,7 +1978,7 @@ void COM_InitFilesystem (void) //johnfit
 	if (i && i < com_argc-1)
 	{
 		com_modified = true;
-		COM_AddGameDirectory (va("%s/%s", com_basedir, com_argv[i + 1]));
+		COM_AddGameDirectory (com_basedir, com_argv[i + 1]);
 	}
 
 	COM_CheckRegistered ();