mountpoint-util: rename fd_is_mount_point() to is_mount_point_at()

fd_* functions in our codebase usually mean fd-specific operations,
while this one actually takes openat()-style params. Rename it
accordingly hence.

src/shared/machine-id-setup.c

@@ -329,7 +329,7 @@ int machine_id_commit(const char *root) {
         if (!etc_machine_id)
                 return log_oom();
 
-        r = fd_is_mount_point(etc_fd, "machine-id", /* flags= */ 0);
+        r = is_mount_point_at(etc_fd, "machine-id", /* flags= */ 0);
         if (r < 0)
                 return log_error_errno(r, "Failed to determine whether %s is a mount point: %m", etc_machine_id);
         if (r == 0) {