mount_nullfs: rename a local variable

The "source" variable was introduced in r26072, probably as the
traditional counterpart to "target".  But the "source"/"target" names
suggest the opposite of their actual meaning.  With ln, for example, the
source is the real file and the target is the newly created link.  In
mount_nullfs the meaning is the opposite: the target is the existing
file system and the source is the newly created mountpoint.  Better to
use "target"/"mountpoint" terminology, which matches the man page.

MFC after:	6 weeks
Sponsored by:	Axcient

sbin/mount_nullfs/mount_nullfs.c

@@ -66,7 +66,7 @@ main(int argc, char *argv[])
 {
 	struct iovec *iov;
 	char *p, *val;
-	char source[MAXPATHLEN];
+	char mountpoint[MAXPATHLEN];
 	char target[MAXPATHLEN];
 	char errmsg[255];
 	int ch, iovlen;
@@ -97,21 +97,21 @@ main(int argc, char *argv[])
 	if (argc != 2)
 		usage();
 
-	/* resolve target and source with realpath(3) */
+	/* resolve target and mountpoint with realpath(3) */
 	if (checkpath(argv[0], target) != 0)
 		err(EX_USAGE, "%s", target);
-	if (checkpath(argv[1], source) != 0)
-		err(EX_USAGE, "%s", source);
+	if (checkpath(argv[1], mountpoint) != 0)
+		err(EX_USAGE, "%s", mountpoint);
 
 	build_iovec(&iov, &iovlen, "fstype", nullfs, (size_t)-1);
-	build_iovec(&iov, &iovlen, "fspath", source, (size_t)-1);
+	build_iovec(&iov, &iovlen, "fspath", mountpoint, (size_t)-1);
 	build_iovec(&iov, &iovlen, "target", target, (size_t)-1);
 	build_iovec(&iov, &iovlen, "errmsg", errmsg, sizeof(errmsg));
 	if (nmount(iov, iovlen, 0) < 0) {
 		if (errmsg[0] != 0)
-			err(1, "%s: %s", source, errmsg);
+			err(1, "%s: %s", mountpoint, errmsg);
 		else
-			err(1, "%s", source);
+			err(1, "%s", mountpoint);
 	}
 	exit(0);
 }