vfs: port 37dbb263's operator-path mknod FIFO/socket fallback

Linear-rebase counterpart of the round-2 merge resolution: 37dbb263 fixed
do_mknod_at()'s operator branch in syscall.c (deleted on this branch) to
fall back to mkfifoat() for a FIFO and EOPNOTSUPP for a nested socket when
mknodat() can't make a special file on the BSDs/macOS/Solaris.  Re-express
it in the VFS_OPERATOR_PATH branch of vfs__mknod_secure() in vfs/mknod.c,
mirroring the secure-relpath branch already in that function.

Tree is byte-identical to the validated round-2 merge result.

vfs/mknod.c

@@ -126,6 +126,19 @@ static int vfs__mknod_secure(const char *pathname, mode_t mode, dev_t dev, int f
 		if (dfd < 0)
 			return -1;
 		ret = mknodat(dfd, bname, mode, dev);
+		if (ret < 0) {
+			/* mknodat() can't make a FIFO/socket on the BSDs/macOS/
+			 * Solaris (EINVAL); retry race-safely on the held dirfd,
+			 * mirroring the secure-relpath path below.  Without this a
+			 * FIFO backup to an operator --backup-dir fails there. */
+#ifdef HAVE_MKFIFOAT
+			if (S_ISFIFO(mode))
+				ret = mkfifoat(dfd, bname, mode);
+			else
+#endif
+			if (S_ISSOCK(mode))
+				errno = EOPNOTSUPP; /* no dirfd-relative socket bind */
+		}
 		e = errno;
 		close(dfd);
 		errno = e;