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/util.h

@@ -188,6 +188,7 @@ extern void pinpoint();
 extern int int_list_cmp(const void* v1, const void* v2);
 
 extern const char* bro_path();
+extern const char* bro_magic_path();
 extern const char* bro_prefixes();
 std::string dot_canon(std::string path, std::string file, std::string prefix = "");
 const char* normalize_path(const char* path);