Fix log level "everything" and make it equal to "debug"

As Holger points out "logging level XXX everything" wasn't working, as
it sets category->loglevel to 0, which is checked in osmo_vlogp() and
will never get logged.
diff --git a/src/logging.c b/src/logging.c
index 1b94ec0..f64fd3c 100644
--- a/src/logging.c
+++ b/src/logging.c
@@ -55,7 +55,7 @@
 #define LOGLEVEL_DEFS	6	/* Number of loglevels.*/
 
 static const struct value_string loglevel_strs[LOGLEVEL_DEFS+1] = {
-	{ 0,		"EVERYTHING" },
+	{ LOGL_DEBUG,	"EVERYTHING" },	/* backwards compatibility */
 	{ LOGL_DEBUG,	"DEBUG" },
 	{ LOGL_INFO,	"INFO" },
 	{ LOGL_NOTICE,	"NOTICE" },