Add a secondary Posix-compatible mode

[willghatch]

Personally, I don't care much about trying to run existing scripts in Rash, but I definitely want to be able to source /etc/profile with it!

This is especially a problem on eg. NixOS, because without sourcing /etc/profile (or copying the relevant bits into a rashrc), Rash inherits Racket's limited environment instead of getting a useful environment... This has been a pain since I switched to NixOS.

Ultimately, though, supporting /etc/profile eventually means there should be a mode that is basically Posix shell support in Racket. Then I guess we could make a mode that is Posix compatible but extended to allow Rash-like things as well.