Log Message:
fake up xorgproto support for native xsrc.  it has some failings
but is likely sufficient:

- add xorgproto support to x11-links, bump version
- add a builtin.mk for xorgproto.  check for the list of packages
  that xorgproto covers, eliding the few that native xsrc never
  installed, and making sure xproto is last.  use the
  BUILTIN_VERSION_SCRIPT method to convert Version: 7.0.31 into
  the necessary 2018.1.

the missing packages are a problem, but since we've never had a
report about it, it's not a big problem.

the things newer xorgproto has that are wanted may be a problem,
but probably only for X11_TYPE!=native builds that don't really
need these.

longer term native xsrc will get xorgproto (and maybe even for
netbsd-8), though i'm not sure it will help any unless we make
our native xsrc install a fake xorgproto.pc that pkgsrc can
find an use -- currently xorgproto installs .pc files for all
the replaced packages, but not one for itself, so the hack of
finding xproto version and switching may still be needed.

ok @wiz.