bsd.port.mk: Move sanity checks later.

[mat]

Summary

Some USES set some variables that a porter may need to be aware and
set some variables too late for sanity checks to do a good job.

This would have caught the KDE5 ports needing the DOCS option in D13651.

[mat]

Wondering if this needs an exp-run.

[antoine]

Running make check-sanity on all ports will be 2 or 3 days faster than an exp-run.

[mat]

true.

[mat]

Solve problems encountered by running:

$ make check-sanity TARGETS=check-sanity LOCALBASE=/var/empty BATCH=

• fixup! Move sanity checks later.
• Remove unused and deprecated variable.
• Rename variable so it does not clash with sanity checks.
• Remove obsolete deprecation.
• Fix invalid license.

[mat]

rebase.

[mat]

Committed in rP479421