Log Message:
rc.subr: update to version 20210517

This is part 2 to support additional platforms with the RC scripts provided in
pkgsrc, in privileged as well as in unprivileged mode, including on NetBSD.

These changes:
- install rc.subr to PKG_SYSCONFDIR if SYSCONFBASE is not /etc
- let RC scripts source rc.subr in PKG_SYSCONFDIR
- effectively allow unprivileged setups to run RC scripts easily
- in addition, give a chance for more platforms to run RC scripts

Tested in privileged and unprivileged modes on NetBSD/amd64, and unprivileged
mode on Darwin/amd64; submitted for review on tech-pkg@.