Check fcache st_uid == geteuid(), not getuid()

Programs like sshd may create or access a ccache with
ruid != user's UID, euid == user's UID.

Set-uid-0 programs (ob reminder: they start life as ruid == user's UID,
euid == 0) shouldn't unintentionally access ccaches.  Therefore we
shouldn't check both of ruid and euid, just euid.