pmbootstrap init: prompt for location of pmaports.git

When running pmbootstrap init, I already have a local clone of pmaports, and would prefer to use that one.

Much like pmbootstrap prompts for a Work Path, it should prompt for pmaports.git.

This would also make pmbootstrap init usable offline in cases where pmaports is already available, aside from easing usage with a diverging checkout of pmaporks.

Current approach

pmbootstrap init unconditionally downloads pmaports. This includes after every run of zap.

Expected approach

pmbootstrap init should allow using my existing pmaports checkout.