syscall: do_mknod_at must mkfifoat for an operator-path FIFO on non-Linux

The operator_path_resolve branch of do_mknod_at() called mknodat() and
returned its result directly, without the FIFO/socket fallback that the
bare-path do_mknod() and the secure-relpath branch below it both have.
mknodat() can make a FIFO only on Linux; on the BSDs/macOS/Solaris it fails
with EINVAL, so creating a special file under an operator-supplied path --
e.g. backing up a FIFO into a --backup-dir -- failed there.  Retry race-safely
with mkfifoat() on the held parent dirfd, and fail a nested socket closed
(EOPNOTSUPP) exactly as the secure-relpath path does.

syscall.c

@@ -1055,6 +1055,19 @@ int do_mknod_at(const char *pathname, mode_t mode, dev_t dev)
 		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;