ubik: Introduce new beacon lock

ubik: Introduce new beacon lock

A new lock is introduced to protect beacon related data when
compiled with pthreads.  A global structure is added containing
the lock itself and the global variables that it protects.

The lock also protects some values in the ubik_server structures:
	lastVoteTime
	lastBeaconSent
	lastVote
	up
	beaconSinceDown

Based on some analysis and design work by Jeffrey Hutzelman

Change-Id: I13f72d32dce71d0686406efcd07b7ea7528722f1
Reviewed-on: http://gerrit.openafs.org/4155
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>