Add support for mksh

[fallwith]

frum init produces an error about not being able to infer the shell when ran under mksh.

From looking at frum's source and documentation, the __frumcd shell function and the eval and frum init commands for bash all seem like they will work just fine with mksh. I added mksh alongside bash and sh and rebuilt frum and everything is working well for me.

I do not currently have access to other ksh derivatives to test with, and I am not sure of the current state of ksh derivative support in the "shell" crate. But this simple change has me up and running and enjoying frum.

changelog: Add support for mksh by leveraging the existing bash/sh shell functionality