logging vty: describe 'logging set-log-mask' command, add OSMO_STRINGIFY_VAL()

Change-Id: I73ac5873ede858da44e1486d8a5c81da1ed5b19f
diff --git a/include/osmocom/core/utils.h b/include/osmocom/core/utils.h
index 0a3805d..5c660cd 100644
--- a/include/osmocom/core/utils.h
+++ b/include/osmocom/core/utils.h
@@ -19,6 +19,8 @@
  * Note: if nested within another preprocessor macro, this will
  * stringify the value of x instead of its name. */
 #define OSMO_STRINGIFY(x) #x
+/*! Stringify the value of a macro x, e.g. a port number. */
+#define OSMO_STRINGIFY_VAL(x) OSMO_STRINGIFY(x)
 /*! Make a value_string entry from an enum value name */
 #define OSMO_VALUE_STRING(x) { x, #x }
 /*! Number of bytes necessary to store given BITS */