[CS6] Port posix-regex egg from CS5

Hi!

I have just released version 0.1.1 of the posix-regex egg, which
includes support for CS6. The egg is already part of the CS5 index,
would it be possible to add it to the CS6 index as well?

See: https://wiki.call-cc.org/eggref/5/posix-regex

Cheers,
Sören