[Proj] datum shift files confusing

[Greg Troxel]

The datum shift files in the ftp server are confusing.  The top page
says to use proj-datumgrid-1.3.zip, but doesn't point out that both

  proj-nad27-1.2.tar.gz
  proj-nadus-1.0.tar.gz

are superceded by proj-datumgrid-1.3.  This is particularly
unintuitive since .tar.gz changed to zip.

On inspection, I concluded that the diff from proj-nadus-10 to
proj-nad27-1.2 resulted in:
  addition of ntv1_can.dat
  addition of null.llv

Going from proj-nad27-1.2 to proj-datumgrid-1.3 resulted in:
  addition of nzgd2kgrid0005.gsb
  changing of README to clarify extent of provided files.

I think it would help to
  provide proj-datumgrid-1.3.tar.gz
  add in parens at end of description of datumgrids
  "(supercedes and extends proj-nadus and nad27)"

The FAQ refers to proj-nad27-1.1 still.

I've updated pkgsrc to use proj-datumgrid-1.3, but thought I'd share
what I figured out in case it's helpful to someone.