Added a ring buffer log target to store the last N log messages.

The log target can be used via log alarms and show alarms.
Why? This feature was proposed/requested at
http://openbsc.osmocom.org/trac/wiki/Tasks/ErrorLogTarget
All messages use the same amount of space, prioritizing simplicity.
diff --git a/include/osmocom/core/logging.h b/include/osmocom/core/logging.h
index 54262b7..fa3365a 100644
--- a/include/osmocom/core/logging.h
+++ b/include/osmocom/core/logging.h
@@ -114,6 +114,7 @@
 	LOG_TGT_TYPE_SYSLOG,	/*!< \brief syslog based logging */
 	LOG_TGT_TYPE_FILE,	/*!< \brief text file logging */
 	LOG_TGT_TYPE_STDERR,	/*!< \brief stderr logging */
+	LOG_TGT_TYPE_STRRB,	/*!< \brief osmo_strrb-backed logging */
 };
 
 /*! \brief structure representing a logging target */
@@ -154,6 +155,10 @@
 		struct {
 			void *vty;
 		} tgt_vty;
+
+		struct {
+			void *rb;
+		} tgt_rb;
 	};
 
 	/*! \brief call-back function to be called when the logging framework