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/log.h

@@ -38,6 +38,18 @@ extern int shmlog_size;
  */
 void init_logging(void);
 
+/**
+ * Opens the logbuffer.
+ *
+ */
+void open_logbuffer(void);
+
+/**
+ * Closes the logbuffer.
+ *
+ */
+void close_logbuffer(void);
+
 /**
  * Set debug logging.
  *