Fix: libcrmcommon: export logfile environment variable if using default

When not explicitly specifying a log, Pacemaker before 2.0.0 would set the
logfile environment variables when checking the corosync configuration for
logging options. Since Pacemaker 2 does not use corosync's logging options
anymore, this step was skipped. However, some resource agents relied on the
environment variables to enable logging, so this introduced a regression.

This fixes it by exporting the default log name when (successfully) used.

lib/common/logging.c

@@ -334,6 +334,11 @@ crm_add_logfile(const char *filename)
     if(is_default) {
         default_fd = fd;
 
+        // Some resource agents will log only if environment variable is set
+        if (daemon_option("logfile") == NULL) {
+            set_daemon_option("logfile", filename);
+        }
+
     } else if(default_fd >= 0) {
         crm_notice("Switching to %s", filename);
         qb_log_ctl(default_fd, QB_LOG_CONF_ENABLED, QB_FALSE);