Subpackages: Update

Hi FreeBSD porters.

Here are some updates about subpackages.

At the moment, subpackages adoption is *on hold*, as a few annoying issues
have been identified and they need some time to be fixed.
A bug about optional dependencies for subpackages has already been fixed
<https://cgit.freebsd.org/ports/commit/Mk?id=064923481f420136c46895d1047344c580be3c25>
(OPT_*_DEPENDS.subpkg was not detected correctly)

The main blockers are:
* `make describe` is not subpackages aware. A fix is in review, but not yet
ready to land. This is also breaking INDEX.
* `make install` doesn't allow for partial installation. A patch is ready
and in review (https://reviews.freebsd.org/D43789) to address it (in
reference to previous email threads)
* poudriere forces the rebuild of subpackages dependency. A PR is open and
we hope to find a solution soon.
Additionally, `USES.subpkg` doesn't work properly at the moment:
dependencies (LIB and RUN) are linked to the main package, instead of the
subpackage.

Because of those issues, some subpackage commits have also been *reverted*,
waiting for the fixes to land, before being committed back.

If you are thinking of adopting subpackages, *please wait* until things get
fixed.
Once those fixes are out and the situation gets more stable, we will focus
on the *documentation*.

Best regards,
pizzamig (on behalf of portmgr)