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.

include/commands.h

@@ -271,4 +271,10 @@ void cmd_rename_workspace(I3_CMD, char *old_name, char *new_name);
  */
 void cmd_bar(I3_CMD, char *bar_type, char *bar_value, char *bar_id);
 
+/*
+ * Implementation of 'shmlog <size>|toggle|on|off'
+ *
+ */
+void cmd_shmlog(I3_CMD, char *argument);
+
 #endif