Fix detection of file system being shared. After this change commands like:

# zfs unshare -a
	# zfs destroy foo/bar
	# zfs rename foo/bar foo/baz

should properly remove exported file systems.

MFC after:	3 days

cddl/contrib/opensolaris/lib/libzfs/common/libzfs_mount.c

@@ -172,6 +172,7 @@ is_shared(libzfs_handle_t *hdl, const char *mountpoint, zfs_share_proto_t proto)
 
 		*tab = '\0';
 		if (strcmp(buf, mountpoint) == 0) {
+#if defined(sun)
 			/*
 			 * the protocol field is the third field
 			 * skip over second field
@@ -194,6 +195,10 @@ is_shared(libzfs_handle_t *hdl, const char *mountpoint, zfs_share_proto_t proto)
 					return (0);
 				}
 			}
+#else
+			if (proto == PROTO_NFS)
+				return (SHARED_NFS);
+#endif
 		}
 	}