fd-util: add wildcard_fd_is_valid() helper and use it tree-wide

Many *at()-style helpers accept a directory fd that may be either a
regular, valid fd (>= 0) or one of the special AT_FDCWD/XAT_FDROOT
wildcard values, and open-code that check in their assertions. Add a
wildcard_fd_is_valid() helper for it and use it tree-wide.

src/libsystemd/sd-id128/id128-util.c

@@ -142,7 +142,7 @@ int id128_read_fd(int fd, Id128Flag f, sd_id128_t *ret) {
 int id128_read_at(int dir_fd, const char *path, Id128Flag f, sd_id128_t *ret) {
         _cleanup_close_ int fd = -EBADF;
 
-        assert(dir_fd >= 0 || IN_SET(dir_fd, AT_FDCWD, XAT_FDROOT));
+        assert(wildcard_fd_is_valid(dir_fd));
         assert(path);
 
         fd = xopenat(dir_fd, path, O_RDONLY|O_CLOEXEC|O_NOCTTY);

src/libsystemd/sd-id128/sd-id128.c

@@ -138,7 +138,7 @@ _public_ int sd_id128_get_machine(sd_id128_t *ret) {
 int id128_get_machine_at(int rfd, sd_id128_t *ret) {
         int r;
 
-        assert(rfd >= 0 || IN_SET(rfd, AT_FDCWD, XAT_FDROOT));
+        assert(wildcard_fd_is_valid(rfd));
 
         r = dir_fd_is_root_or_cwd(rfd);
         if (r < 0)