Do not DWIM in userpath library under strict mode.

This should force git-daemon administrator's job a bit harder
because the exact paths need to be given in the whitelist, but
at the same time makes the auditing easier.

This moves validate_symref() from refs.c to path.c, because we
need to link path.c with git-daemon for its "enter_repo()", but
we do not want to link the daemon with the rest of git libraries
and its requirements.

Signed-off-by: Junio C Hamano <junkio@cox.net>