mmap: accept non-PROT_WRITE MAP_SHARED mappings

Currently we don't accept writable file mmap()s, as there is no
system in place to guarantee dirty buffers would make it back to
disk. But we can actually accept MAP_SHARED for PROT_READ mappings,
meaning the ranges aren't writable at all (and no private copy is
made as with MAP_PRIVATE), as it turns out a fairly large class of
usage.

	. fail writable MAP_SHARED mappings at runtime
	. reduces some minix-specific patches
	. lets binutils gold build on minix without further patching

Change-Id: If2896c0a555328ac5b324afa706063fc6d86519e

sys/sys/mman.h

@@ -68,9 +68,7 @@ typedef	__off_t		off_t;		/* file offset */
  * Flags contain sharing type and options.
  * Sharing types; choose one.
  */
-#if !defined(__minix)
 #define	MAP_SHARED	0x0001	/* share changes */
-#endif /* !defined(__minix) */
 #define	MAP_PRIVATE	0x0002	/* changes are private */
 
 #ifdef _KERNEL