bsdinstall: add country/regulatory domain configuration dialog

[avos]

Summary

Allow to setup country and/or regulatory domain before scanning (since channel list may be affected by current regulatory restrictions this may impact scan results).

Should be applied before D6228 (enforces stricter limitations by default).

Test Plan

Tested with local and network installations; no visible regressions.

[nwhitehorn]

I have no objections to this from the installer end. Thanks! I'll leave it to others to comment on the wireless aspects (and ultimately approve).

[avos]

ping