Add a logging category for TLS pre-master secrets

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.