main/ppp: revert lock dir to /var/lock

Backport upstream fix for pppd failing to start when lock dir does not exist (by reverting the lock dir to somewhere that should always exist)

This fixes using pppd from something else than the main pppd service (e.g. networkmanager of ifupdown-ng), as these would not go through the checkpath hook, and pppd can now run without /run/ppp

Link: https://github.com/ppp-project/ppp/pull/435
Fixes #15145 (closed)