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.

parser-specs/commands.spec

@@ -19,6 +19,7 @@ state INITIAL:
   'exit' -> call cmd_exit()
   'restart' -> call cmd_restart()
   'reload' -> call cmd_reload()
+  'shmlog' -> SHMLOG
   'border' -> BORDER
   'layout' -> LAYOUT
   'append_layout' -> APPEND_LAYOUT
@@ -62,6 +63,12 @@ state EXEC:
   command = string
       -> call cmd_exec($nosn, $command)
 
+# shmlog <size>|toggle|on|off
+state SHMLOG:
+  # argument may be a number
+  argument = string
+    -> call cmd_shmlog($argument)
+
 # border normal|none|1pixel|toggle|1pixel
 state BORDER:
   border_style = 'normal', 'pixel'