LWLocks during a panic exit.  This avoids the possible self-deadlock pointed
out by Qingqing Zhou.  Also, I noted that an error during LoadFreeSpaceMap()
or BuildFlatFiles() would result in exit(0) which would leave the postmaster
thinking all is well.  Added a critical section to ensure such errors don't
allow startup to proceed.

Backpatched to 8.1.  The 8.0 code is a bit different and I'm not sure if the
problem exists there; given we've not seen this reported from the field, I'm
going to be conservative about backpatching any further.