fs-util: teach xopenat_full() about XO_SOCKET and XO_TRIGGER_AUTOMOUNT

fs-util: teach xopenat_full() about XO_SOCKET and XO_TRIGGER_AUTOMOUNT

XO_SOCKET asks for the opened inode to be verified as a socket, analogous to
the existing XO_REGULAR. A new fd_verify_socket() helper is added to
stat-util to perform the check on an opened fd.

XO_TRIGGER_AUTOMOUNT asks O_PATH opens to trigger automounts by going via
open_tree() without OPEN_TREE_CLONE, falling back to openat() if the kernel
or sandbox rejects open_tree().