Turned out that as "extension" (which is really more like a pattern
that matches the whole file) was even used with patterns like "*.*"
so we do proper matching using fnmatch() now - which is even easier
than the old way.
Now deleting savegames on POSIX works.
If the window loses focus com_pause is set to 1, when it regains focus
it's set to 0.
The behaviour on Win32 stayed the same (the implementation is less
hacky though) and Linux now matchces that.
In Sys_ListFiles() the extension was abused to match whole
filenames (of savegames), not just their extensions - that
didn't work with the POSIX backend, now it does.
Sys_ListFiles() now also supports "*" as a wildcard that matches
all files - needed for deleting savegames.
However, things like "bl*" or "*ub" don't work.
While at it, I replaced readdir() with the thread-safe readdir_r()
