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

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.
  *

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)
  *