libowfat: reimplementation of the libdjb under GPL

* Moved from `headonly/opentracker` to core: homebrew/homebrew-headonly#19;
* Bumped version from `0.28` to `0.29`;
* Added checksum;
* Added `--HEAD` build.

[Macports](https://www.macports.org/ports.php?by=name&substr=libowfat), [FreeBSD](http://www.freshports.org/devel/libowfat/), Ubuntu, etc have it. Also judging by freshports there are number of packages that depend on `libowfat` other than `opentracker` (e.g. `ncp`, `tinyldap` and `gatling`, though all from same author) which will simplify their porting to homebrew.