man.sr.ht: upgrade to 0.15.12

*.sr.ht: move nginx files

man.sr.ht-nginx: add sr.ht-nginx dependency

man.sr.ht: add -nginx subpackage

man.sr.ht: overhaul init files

man.sr.ht: add py3-scmsrht dependency

man.sr.ht: normalize init scripts

Add py3-yaml to man.sr.ht dependencies

Add -openrc to man.sr.ht

Update man.sr.ht.post-upgrade

Also moves alembic dependency to py3-srht

Update man.sr.ht.init

Add man.sr.ht package