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

libexec/ftpd/ftpd.c

@@ -2232,7 +2232,6 @@ send_data_with_mmap(int filefd, int netfd, const struct stat *st, int isdata)
 		(void)gettimeofday(&then, NULL);
 	} else
 		bufrem = winsize;
-#if !defined(__minix)
 	while (1) {
 		mapsize = MIN(filesize - off, winsize);
 		if (mapsize == 0)
@@ -2244,10 +2243,14 @@ send_data_with_mmap(int filefd, int netfd, const struct stat *st, int isdata)
 				goto try_read;
 			return (SS_FILE_ERROR);
 		}
+#ifndef __minix
 		(void) madvise(win, mapsize, MADV_SEQUENTIAL);
+#endif
 		error = write_data(netfd, win, mapsize, &bufrem, &then,
 		    isdata);
+#ifndef __minix
 		(void) madvise(win, mapsize, MADV_DONTNEED);
+#endif
 		munmap(win, mapsize);
 		if (urgflag && handleoobcmd())
 			return (SS_ABORTED);
@@ -2257,7 +2260,6 @@ send_data_with_mmap(int filefd, int netfd, const struct stat *st, int isdata)
 	}
 	return (SS_SUCCESS);
 
-#endif /* !defined(__minix) */
  try_read:
 	return (send_data_with_read(filefd, netfd, st, isdata));
 }