debian/patches: cherry-pick upstream LDFLAGS fix

Allow hardening options set using the standard mechanism.
debian/rules: add hardening options
debian/changelog: squash, begin ~mentorsN tagging

Following Paul Tagliamonte's advice, append ~mentors1, reset Debian major
revision to 1, and never go to -1 or anything above. This may or may not
increment to 1.1.0-1~mentors1 or beyond depending on what sponsors want me
to do. For now, I need to progress the packaging, and as such I'll continue
accumulating changes into a single, big UNRELEASED entry.

This implies [notyetnotreally]Debian revision numbers which now have gone
backwards, but I am doing this to fix my mistake. Hopefully the exposure is
limited to expert users who test things from Mentors (and mentors doesn't
make binaries available, so that's okay(?)).

There are *no nice solutions* to this; I painted myself into a corner.
Sorry. I don't wish to use an epoch number as a workaround because that
would only raise more eyebrows.

Coincides with blank-slating the mypaint packages on mentors.debian.net,
and profound apologies to anyone watching this package evolve: you'll just
have to remove or downgrade any previously installed packages from mentors.