Revert lock path to /var/lock (#435)

lock dir changed on linux from /var/lock to /run/pppd/lock with
pppd-2.5.0, which makes pppd fail to start if the distribution does not
pre-create the directory.

This reverts it back to /var/lock.

The paths for other OS should be identical as LOCALSTATEDIR should be
/var, but also revert them back as well just in case.
Since the variable is no longer used remove it from makefiles.

Fixes: 66a8c74c3f73 ("Let ./configure control the paths for pppd")
Fixes: #419
Signed-off-by: Dominique Martinet <dominique.martinet@atmark-techno.com>
Co-authored-by: Dominique Martinet <dominique.martinet@atmark-techno.com>