mail-mta/nullmailer: revbump and more

mail-mta/nullmailer: revbump and more

Revbump, EAPI 7, add myself as maintainer.
Closes: https://bugs.gentoo.org/654124

Add ~arm keyword.
Closes: https://bugs.gentoo.org/664514

Fix permission for /var/spool/nullmailer/failed,
credits to John R. Graham.
Closes: https://bugs.gentoo.org/622738

Keep dir /var/spool/nullmailer/failed.
Closes: https://bugs.gentoo.org/651288

No not depend on sys-apps/groff.
Closes: https://bugs.gentoo.org/604316

Tell if 'emerge --config' gets empty FQDN or domainname,
and remind to tweak /etc/nullmailer/remotes.
Closes: https://bugs.gentoo.org/668104

Block with USE=mta against msmtp,opensmtpd,ssmtp.
Closes: https://bugs.gentoo.org/631654

Package-Manager: Portage-2.3.51, Repoman-2.3.11
Signed-off-by: Michael Haubenwallner <haubi@gentoo.org>