dosfns, int2f: make SHARE uninstallable

Refer to https://github.com/FDOS/share/issues/10

kernel/dosfns.c

@@ -1395,8 +1395,10 @@ BOOL IsShareInstalled(BOOL recheck)
   extern unsigned char ASMPASCAL share_check(void);
   if (recheck == FALSE)
     return share_installed;
-  if (!share_installed && share_check() == 0xff)
+  if (share_check() == 0xff)
     share_installed = TRUE;
+  else
+    share_installed = FALSE;
   return share_installed;
 }
 

kernel/int2f.asm

@@ -250,6 +250,7 @@ IntDosCal:
 SHARE_CHECK:
 		mov	ax, 0x1000
 		int	0x2f
+		test	ax, "US"	; Uninstallable SHARE signature
 		ret
            
 ;           DOS calls this to see if it's okay to open the file.