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
