mksh: moved to github

Signed-off-by: Thorsten Glaser <tg@mirbsd.org>
mksh: SECURITY update to R50d

- new upstream version
- update my eMail address
- add maintainer and licence tag

Signed-off-by: Thorsten Glaser <tg@mirbsd.org>
Merge pull request #411 from mirabilos/mksh

move mksh to github; update it