mail-filter/postforward: Initial ebuild

mail-filter/postforward: Initial ebuild

Postforward is a mail forwarding utility which aims to compliment the
Postfix Sender Rewriting Scheme daemon (PostSRSd) by allowing only
certain addresses to be forwarded while leaving leaving non-forwarded
mail unaltered by PostSRSd.

 - Create initial ebuild
 - Work around upstream not putting their .go files in src/ which
   makes "go build" angry
 - Work around golang-build complaining about pkg not existing
   and just do the install by hand
 - Patch default sendmail path to respect EPREFIX and use the
   standard postfix install location instead of depending on
   it being in the path
 - Implement requests made by monsueirp on GitHub:
   https://github.com/gentoo/gentoo/pull/5910#pullrequestreview-83093569
 - Implement requests made by mgorny on GitHub:
   https://github.com/gentoo/gentoo/pull/5910#pullrequestreview-103921727

Package-Manager: Portage-2.3.8, Repoman-2.3.3
Closes: https://bugs.gentoo.org/650398
Closes: https://github.com/gentoo/gentoo/pull/5910