configuration files.
From now on, mk.conf is first searched under the value passed to the
sysconfdir argument (which defaults to ${prefix}/etc, to match pkgsrc's
PKG_SYSCONFDIR default value). If not found, /etc/mk.conf is tried, to
not break existing installations which have the file in that location.
This is done to help with non-root installations of pkgsrc.
Also change pkgsrc's PKG_SYSCONFBASE value to match what is given to
sysconfdir, for consistency.
While doing this, rename $opsys.own.mk files to $opsys.own.mk.in for clarity,
as they now need sed replacements to work (i.e., the SYSCONFDIR stuff).
