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/x.h

@@ -108,6 +108,12 @@ void x_raise_con(Con *con, bool above_all);
  */
 void x_set_name(Con *con, const char *name);
 
+/**
+ * Set up the SHMLOG_PATH atom.
+ *
+ */
+void update_shmlog_atom(void);
+
 /**
  * Sets up i3 specific atoms (I3_SOCKET_PATH and I3_CONFIG_PATH)
  *