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/Baseline/scripts.base.protocols.syslog.missing-pri/syslog.log

@@ -0,0 +1,10 @@
+#separator \x09
+#set_separator	,
+#empty_field	(empty)
+#unset_field	-
+#path	syslog
+#open	2019-03-15-01-41-39
+#fields	ts	uid	id.orig_h	id.orig_p	id.resp_h	id.resp_p	proto	facility	severity	message
+#types	time	string	addr	port	addr	port	enum	string	string	string
+1552584410.781231	CHhAvVGS1DHFjwGM9	192.168.2.118	60786	192.168.2.21	514	udp	UNSPECIFIED	UNSPECIFIED	This is not really a syslog message #173538 1552584410.781186
+#close	2019-03-15-01-41-39

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