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.

magic/diff

@@ -0,0 +1,25 @@
+# See COPYING file in this directory for original libmagic copyright.
+#------------------------------------------------------------------------------
+# $File: diff,v 1.13 2012/06/16 14:43:36 christos Exp $
+# diff:  file(1) magic for diff(1) output
+#
+0	search/1	diff\ 		diff output text
+!:mime	text/x-diff
+0	search/1	***\ 		diff output text
+!:mime	text/x-diff
+0	search/1	Only\ in\ 	diff output text
+!:mime	text/x-diff
+0	search/1	Common\ subdirectories:\ 	diff output text
+!:mime	text/x-diff
+
+0	search/1	Index:		RCS/CVS diff output text
+!:mime	text/x-diff
+
+# unified diff
+0	search/4096	---\ 
+>&0	search/1024 \n
+>>&0	search/1 +++\ 
+>>>&0	search/1024 \n
+>>>>&0	search/1 @@	unified diff output text
+!:mime	text/x-diff
+!:strength + 90