[3.17] replace busybox ash with another /bin/sh

We've stared deep into the void of the busybox ash parsing code and found it staring back. Among other things we've found:

• mempcpy used in a situation where memmove must be used
• a serious shell substitution bug
• attempting to zero memory used by AST nodes in the stack code makes the whole thing violently explode, which should not ever happen in an otherwise correct codebase

This issue is about proposing a replacement /bin/sh. We want something similar to what we have now, but with a cleaner codebase, probably mksh or something.