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.

src/main.cc

@@ -206,6 +206,7 @@ void usage()
 	fprintf(stderr, "    --use-binpac                   | use new-style BinPAC parsers when available\n");
 
 	fprintf(stderr, "    $BROPATH                       | file search path (%s)\n", bro_path());
+	fprintf(stderr, "    $BROMAGIC                      | libmagic mime magic database search path (%s)\n", bro_magic_path());
 	fprintf(stderr, "    $BRO_PREFIXES                  | prefix list (%s)\n", bro_prefixes());
 	fprintf(stderr, "    $BRO_DNS_FAKE                  | disable DNS lookups (%s)\n", bro_dns_fake());
 	fprintf(stderr, "    $BRO_SEED_FILE                 | file to load seeds from (not set)\n");