Support for setting base system build options via dialog(1)

[larafercue_gmail.com]

Summary

Through this changes the options to build the base system can be established via a shell dialog
as the ones used for the ports.

Test Plan

Run the commands "make config" to create a new configuration, check the /etc/src.conf and
/etc/src-env.conf files which should have the right options activated or deactivated and use "make rmconfig"
to delete these files.

[larafercue_gmail.com]

Reimplementation of the patch

• The previous patch had a bug that I could not find, so I took the

oportunity to redesign the approach and re-implement how the command
make config should work.

[larafercue_gmail.com]

It disapeared from the Ideas Wiki page, it might mean that no one cares about it. If required I could work on it again.