MIME: Cap nested MIME analysis depth to 100

OSS-Fuzz managed to produce a MIME multipart message construction with
thousands of nested entities (or that's what Zeek makes out of it anyhow).
Prevent such deep analysis by capping at a nesting depth of 100,
preventing unnecessary resource usage. A new weird named exceeded_mime_max_depth
is reported when this limit is reached.

This change reduces the runtime of the OSS-Fuzz reproducer from ~45 seconds
to ~2.5 seconds.

The test PCAP was produced from a Python script using the email package
and sending the rendered version via POST to a HTTP server.

Closes #208

scripts/base/init-bare.zeek

@@ -2780,6 +2780,16 @@ export {
 } # end export
 
 
+module MIME;
+export {
+	## Stop analysis of nested multipart MIME entities if this depth is
+	## reached. Setting this value to 0 removes the limit.
+	const max_depth = 100 &redef;
+
+} # end export
+
+
+
 module MOUNT3;
 export {