Export string constants and annotate exported function implementations

Moves scattered string constants to externs.m, and removes obsolete code replacing constant strings.

Source/Additions/GSMime.m

@@ -7932,7 +7932,7 @@ appendString(NSMutableData *m, NSUInteger offset, NSUInteger fold,
 @end
 
 
-NSString* const GSMimeErrorDomain = @"GSMimeErrorDomain";
+GS_DECLARE NSString* const GSMimeErrorDomain = @"GSMimeErrorDomain";
 
 typedef	enum	{
   TP_IDLE,

Source/Additions/NSDebug+GNUstepBase.m

@@ -29,7 +29,7 @@
 #import "common.h"
 #import "GNUstepBase/NSDebug+GNUstepBase.h"
 
-NSString*
+GS_DECLARE NSString*
 GSDebugFunctionMsg(const char *func, const char *file, int line, NSString *fmt)
 {
   NSString *message;
@@ -39,7 +39,7 @@ GSDebugFunctionMsg(const char *func, const char *file, int line, NSString *fmt)
   return message;
 }
 
-NSString*
+GS_DECLARE NSString*
 GSDebugMethodMsg(id obj, SEL sel, const char *file, int line, NSString *fmt)
 {
   NSString	*message;

Source/Additions/NSStream+GNUstepBase.m

@@ -27,18 +27,6 @@
 #import "GNUstepBase/NSObject+GNUstepBase.h"
 #import "GNUstepBase/NSStream+GNUstepBase.h"
 
-/* Code used in GNUstepBase
- */
-NSString * const GSStreamLocalAddressKey
-  = @"GSStreamLocalAddressKey";
-NSString * const GSStreamLocalPortKey
-  = @"GSStreamLocalPortKey";
-NSString * const GSStreamRemoteAddressKey
-  = @"GSStreamRemoteAddressKey";
-NSString * const GSStreamRemotePortKey
-  = @"GSStreamRemotePortKey";
-
-
 /* The remaining code is specific to the Apple Foundation
  */
 #if	!defined(GNUSTEP)