Add NCP::max_frame_size tuning option

This helps prevent excessive allocations based on message lengths
taken from NCP headers.

scripts/base/init-bare.bro

@@ -4396,6 +4396,12 @@ export {
 	const max_frag_data = 30000 &redef;
 }
 
+module NCP;
+export {
+	## The maximum number of bytes to allocate when parsing NCP frames.
+	const max_frame_size = 65536 &redef;
+}
+
 module GLOBAL;
 
 ## Seed for hashes computed internally for probabilistic data structures. Using