Makefile.inc1: Add new buildetc and installetc targets

These implement the steps that etcupdate (and mergemaster) need in order
to build their pristine trees, avoiding the need to use internal targets
and variables. Additionally, buildetc includes the steps needed to build
with BUILD_WITH_STRICT_TMPPATH enabled, since we need some host tools to
be available during the various build and install steps.

Reviewed by: jhb, imp
Differential Revision: https://reviews.freebsd.org/D41205