Implement debuglog command

Add debuglog command that takes toggle|on|off. Add get_debug_logging()
to be able to toggle. Make t/187-commands-parser.t expect 'debuglog'.
Document the debuglog command in userguide.

parser-specs/commands.spec

@@ -20,6 +20,7 @@ state INITIAL:
   'restart' -> call cmd_restart()
   'reload' -> call cmd_reload()
   'shmlog' -> SHMLOG
+  'debuglog' -> DEBUGLOG
   'border' -> BORDER
   'layout' -> LAYOUT
   'append_layout' -> APPEND_LAYOUT
@@ -69,6 +70,11 @@ state SHMLOG:
   argument = string
     -> call cmd_shmlog($argument)
 
+# debuglog toggle|on|off
+state DEBUGLOG:
+  argument = 'toggle', 'on', 'off'
+    -> call cmd_debuglog($argument)
+
 # border normal|none|1pixel|toggle|1pixel
 state BORDER:
   border_style = 'normal', 'pixel'