Merge branch 'trac4002' Save error string before use

Save the error string before using it to minimize the possibility of
it changing between when it gets generated and reviewed and when it
gets logged.

src/lib/util/signal_set.cc

@@ -178,9 +178,10 @@ SignalSet::add(const int sig) {
     sa.sa_handler = internalHandler;
     sigfillset(&sa.sa_mask);
     if (sigaction(sig, &sa, 0) < 0) {
+        const char* errmsg = strerror(errno);
         erase(sig);
         isc_throw(SignalSetError, "failed to register a signal handler for"
-                  " signal " << sig << ": " << strerror(errno));
+                  " signal " << sig << ": " << errmsg);
     }
 }