Implement shmlog command

Implement shmlog command

Add shmlog command that takes <size>|toggle|on|off. Separate logbuffer
management into open_logbuffer() and close_logbuffer(). Make
t/187-commands-parser.t expect 'shmlog'. Add update_shmlog_atom() to
update the SHMLOG_PATH. Document the shmlog command in userguide.