cifs.upcall: scan /run/user/${UID} for ccaches, too

cifs.upcall: scan /run/user/${UID} for ccaches, too

When scanning for credential caches, check the user's directory under
/run/user first, then fall back to /tmp as we have previously.  Because
we now call find_krb5_cc() twice (once for each directory), we move its
state to be outside of the function.  We also add a substitution
mechanism to make the process of resolving the location of the user's
home directory before searching it a bit more explicable.