mountpoint-util: add new helper name_to_handle_at_try_fid()

Newer kernels support a new flag for name_to_handle_at(): AT_HANDLE_FID.
This flag is supposed to return an identifier for an inode that we can
use for checking inode identity. It's supposed to be a replacement for
checking .st_ino which doesn't work anymore today because inode numbers
are no longer unique on file systems (not on overlayfs, and not on btrfs
for example). Hence, be a good citizen and add infrastructure to support
AT_HANDLE_FID. Unfortunately that doesn't work for old kernels, hence
add a fallback logic: if we can use the flag, use it. If we cannot use
name_to_handle_at() without it, which might give us a good ID too. But
of course tha tcan fail as well, which callers have to check.

src/basic/mountpoint-util.h

@@ -39,6 +39,7 @@
 bool is_name_to_handle_at_fatal_error(int err);
 
 int name_to_handle_at_loop(int fd, const char *path, struct file_handle **ret_handle, int *ret_mnt_id, int flags);
+int name_to_handle_at_try_fid(int fd, const char *path, struct file_handle **ret_handle, int *ret_mnt_id, int flags);
 
 bool file_handle_equal(const struct file_handle *a, const struct file_handle *b);