Introduce an EXPERIMENTAL option for both src.conf(5) and the kernel.

In the last decade(s) we have seen both short term or long term projects
committed to the tree which were considered or even marked "experimental".
While out-of-tree development has become easier than it used to be in
CVS times, there still is a need to have the code shipping with HEAD but
not enabled by default.

While people may think about VIMAGE as one of the recent larger, long term
projects, early protocol implementations (before they are standardised)
are others. (Free)BSD historically was one of the operating systems
which would have running code at early stages and help develop and
influence standardisation and the industry.

Give developers an opportunity to be more pro-active for early adoption
or running large scale code changes stumbling over each others but not
the user's feet. I have not added the option to NOTES in order to avoid
breaking supported option builds, which require constant compile testing.

Discussed with: people in the corridor