Making NO_CLEAN configurable through src.conf, possibly default

[Ed Maste]

For several years I've built with NO_CLEAN, and would like us to
consider making it the default in the future. It used to be we
regularly made changes in the tree that required a clean build
(because of missing explicit dependencies, or because of limitations
in the way make automatically tracks dependencies), but for the last
couple of years we've added explicit workarounds when that happens.

As a first step I've proposed having it be set via src.conf WITH_CLEAN
/ WITHOUT_CLEAN knobs, in review https://reviews.freebsd.org/D22762.
What do folks think (of WITH_/WITHOUT_CLEAN and of making it the default)?