New formulas: Plan9's Unix Software

Some formulas for Plan9's other libraries and utilities, aside from the ones in plan9port.
Includes libfmt, libbio, libregexp9, and mk.
Looks like this is included in `plan9port`, too:

```
$ brew ls plan9port -v | grep regex
/usr/local/Cellar/plan9port/20140306/libexec/include/regexp.h
/usr/local/Cellar/plan9port/20140306/libexec/include/regexp9.h
/usr/local/Cellar/plan9port/20140306/libexec/lib/libregexp9.a
...
```