FileAnalysis: add custom libmagic database.

- It's derived from the magic database of libmagic 5.14, but with most
  everything not related to mime types removed.

- The custom database is always used by default for mime detection, but
  the more verbose file type detection will fall back on the default
  libmagic installation's database.  The result is: mime type strings
  are now guaranteed to be consistent across platforms, but the verbose
  file type descriptions are not.

- The custom database gets installed in $prefix/share/bro/magic, and
  should even be extensible if files with new patterns are added inside
  the directory.

- The search path for the mime magic database can be controlled via
  BROMAGIC environment variable.

- Remove mime_desc field from ftp.log.

- Stop using the mime/file type canonifier with unit tests.

- libmagic >= 5.04 is now a requirement.

testing/btest/Baseline/scripts.base.protocols.http.http-extract-files/http.log

@@ -6,5 +6,5 @@
 #open	2013-03-22-14-38-28
 #fields	ts	uid	id.orig_h	id.orig_p	id.resp_h	id.resp_p	trans_depth	method	host	uri	referrer	user_agent	request_body_len	response_body_len	status_code	status_msg	info_code	info_msg	filename	tags	username	password	proxied	mime_type	md5	extraction_file
 #types	time	string	addr	port	addr	port	count	string	string	string	string	string	count	count	count	string	count	string	string	table[enum]	string	string	table[string]	string	string	string
-1128727435.634189	arKYeMETxOg	141.42.64.125	56730	125.190.109.199	80	1	GET	www.icir.org	/	-	Wget/1.10	0	9130	200	OK	-	-	-	(empty)	-	-	-	text/html; charset=us-ascii	-	http-item-BFymS6bFgT3-0.dat
+1128727435.634189	arKYeMETxOg	141.42.64.125	56730	125.190.109.199	80	1	GET	www.icir.org	/	-	Wget/1.10	0	9130	200	OK	-	-	-	(empty)	-	-	-	text/html	-	http-item-BFymS6bFgT3-0.dat
 #close	2013-03-22-14-38-28