Change the way the plex lock mutexes work. Previously they were part

Change the way the plex lock mutexes work.  Previously they were part
of the struct plex, which tore apart the mutex linked lists when the
plex table was expanded.  Now we maintain a pool of mutexes (currently
32) to be shared by all plexes.  This is still a lot better than the
splhigh() method used in other architectures.

expand_table: Add parameters file and line if we're debugging.

Approved by: re (jhb)