Add a logging category for TLS pre-master secrets

TLS pre-master secrets will be dumped to disk using the logging
framework provided by libisc.  Add a new logging category for this type
of debugging data in order to enable exporting it to a dedicated
channel.  Derive the name of the new category from the name of the
relevant environment variable, SSLKEYLOGFILE.

doc/arm/logging-categories.rst

@@ -97,6 +97,9 @@
 ``spill``
     Queries that have been terminated, either by dropping or responding with SERVFAIL, as a result of a fetchlimit quota being exceeded.
 
+``sslkeylog``
+    TLS pre-master secrets (for debugging purposes).
+
 ``trust-anchor-telemetry``
     Trust-anchor-telemetry requests received by ``named``.
 

lib/isc/include/isc/log.h

@@ -167,8 +167,9 @@ extern isc_logmodule_t	 isc_modules[];
  * Do not log directly to DEFAULT.  Use another category.  When in doubt,
  * use GENERAL.
  */
-#define ISC_LOGCATEGORY_DEFAULT (&isc_categories[0])
-#define ISC_LOGCATEGORY_GENERAL (&isc_categories[1])
+#define ISC_LOGCATEGORY_DEFAULT	  (&isc_categories[0])
+#define ISC_LOGCATEGORY_GENERAL	  (&isc_categories[1])
+#define ISC_LOGCATEGORY_SSLKEYLOG (&isc_categories[2])
 /*@}*/
 
 #define ISC_LOGMODULE_SOCKET	(&isc_modules[0])

lib/isc/log.c

@@ -181,6 +181,7 @@ isc_logcategory_t isc_categories[] = { { "default", 0 }, /* "default
 							    must come
 							    first. */
 				       { "general", 0 },
+				       { "sslkeylog", 0 },
 				       { NULL, 0 } };
 
 /*!