change lock format to let us detect and break stale locks.
old style: symlink to pid
new style: symlink to hostname:pid
if lock code finds new-style lock, it breaks lock if locking pid is on
same machine and pid is not alive.
otherwise, lock is left alone. this makes locking code safe with
old-style locks and with locks on other machines.
new code makes server part of mercurial more robust in case machine
crashes, power fails, or crazy user does kill -9.