mail/py-pymilter: update to 1.0.6, add options, rename

Take maintainership.

Move port from mail/py-milter to mail/py-pymilter to match the name of
the PyPI package being ported.

Update to version 1.0.6

Use ${PORTSDIR}/mail/sendmail/bsd.milter.mk for consistency with most
other milter ports.

Add options to force selection between the mail/libmilter port and the
libmilter.so bundled with the base FreeBSD-sendmail package. Prefer
the mail/libmilter port on general principles: given that all major
MTA software supports the milter interface, it should be possible to
install milters from pkg(8) without forcing the FreeBSD-sendmail to be
installed.

Changes: https://github.com/sdgathman/pymilter/compare/pymilter-1.0.5...pymilter-1.0.6
Approved by: Gerhard Schmidt (former maintainer)
PR: 286630