Add WITH_/WITHOUT_CLEAN option to replace NO_CLEAN

This allows use of the standard src.conf configuration for controlling
whether the tree is cleaned before build or not. The default is still
to clean.

Setting either NOCLEAN or NO_CLEAN will mention the new src.conf option.
NOCLEAN remains a .warning, while for now NO_CLEAN is .info.

Reviewed by: bdrewery (earlier version)
MFC after: 1 month
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D22762