Improve ERSPAN Type III support

Added check for the optional sub-header in ERSPAN Type III as well
as additional truncation checks to the GRE parsing logic in general.
Also added a unit test for ERSPAN Type II.

testing/btest/Baseline/core.erspanII/conn.log

@@ -0,0 +1,10 @@
+#separator \x09
+#set_separator	,
+#empty_field	(empty)
+#unset_field	-
+#path	conn
+#open	2019-01-17-23-57-23
+#fields	ts	uid	id.orig_h	id.orig_p	id.resp_h	id.resp_p	proto	service	duration	orig_bytes	resp_bytes	conn_state	local_orig	local_resp	missed_bytes	history	orig_pkts	orig_ip_bytes	resp_pkts	resp_ip_bytes	tunnel_parents
+#types	time	string	addr	port	addr	port	enum	string	interval	count	count	string	bool	bool	count	string	count	count	count	count	set[string]
+1402723255.667881	ClEkJM2Vm5giqnMf4h	23.0.0.2	8	23.0.0.3	0	icmp	-	0.001727	144	144	OTH	-	-	0	-	2	200	2	200	CHhAvVGS1DHFjwGM9
+#close	2019-01-17-23-57-23

testing/btest/Baseline/core.erspanII/tunnel.log

@@ -0,0 +1,10 @@
+#separator \x09
+#set_separator	,
+#empty_field	(empty)
+#unset_field	-
+#path	tunnel
+#open	2019-01-17-23-57-23
+#fields	ts	uid	id.orig_h	id.orig_p	id.resp_h	id.resp_p	tunnel_type	action
+#types	time	string	addr	port	addr	port	enum	enum
+1402723255.667881	CHhAvVGS1DHFjwGM9	2.2.2.2	0	1.1.1.1	0	Tunnel::GRE	Tunnel::DISCOVER
+#close	2019-01-17-23-57-23

testing/btest/core/erspanII.bro

@@ -0,0 +1,6 @@
+# @TEST-EXEC: bro -C -b -r $TRACES/erspanII.pcap %INPUT
+# @TEST-EXEC: btest-diff tunnel.log
+# @TEST-EXEC: btest-diff conn.log
+
+@load base/frameworks/tunnels
+@load base/protocols/conn