lintpkgsrc: Update pkgtools/lintpkgsrc to 4.94

pkgsrc changes:
- Use SUBST_VARS instead of SUBST_SED to adjust the several variables in
  files/lintpkgsrc.pl and avoid some make contortionism in the remaining
  SUBST_SED

Changes:
- PKG_SYSCONFDIR can be different to ${PREFIX}/etc, instruct lintpkgsrc
  about that to possibly pick mk.conf when an /etc/mk.conf file is not
  available

Discussed on pkgsrc-changes@ with <adam> and <jperkin>.
Thanks to both <adam> and <jperkin> for feedback, suggestions and review!

(leot)