Make Syslog analyzer accept messages that omit Priority

Essentially, it will now process/parse priority values if they are
there, or else just accept whatever remaining data/text is there as the
syslog message.  Reasoning is that there's syslog producers out there
that may have simply forgotten/neglected to send the priority value
and technically won't conform to what the standard says, though we can
infer the intent (some syslog consumers already may do similarly, but
I didn't verify).

testing/btest/scripts/base/protocols/syslog/missing-pri.bro

@@ -0,0 +1,4 @@
+# @TEST-EXEC: bro -r $TRACES/syslog-missing-pri.trace %INPUT
+# @TEST-EXEC: btest-diff syslog.log
+
+@load base/protocols/syslog