openbsd: properly set daemon workdir, use default rc_start, set MIX_ENV in login.conf

Setting the MIX_ENV variable in rc_pre() isn't possible, because the
environment doesn't persist between rc_pre and rc_start(). This way we
can also ditch the custom rc_start() function in favor of the default
one which is just:

rc_start() {
	rc_exec "${daemon} ${daemon_flags}
}

docs/installation/openbsd_en.md

@@ -69,7 +69,7 @@ pleroma:\
 	:datasize=1536M:\
 	:openfiles-max=4096:\
 	:openfiles-cur=1024:\
-	:setenv=LC_ALL=en_US.UTF-8,VIX_COMPILATION_MODE=PLATFORM_PROVIDED_LIBVIPS:\
+	:setenv=LC_ALL=en_US.UTF-8,VIX_COMPILATION_MODE=PLATFORM_PROVIDED_LIBVIPS,MIX_ENV=prod:\
 	:tc=daemon:
 ```