Updated install-font.sh

[Peter Schaffter]

The install-font script at

  https://www.schaffter.ca/mom/mom-05.html#install-font

isn't officially part of groff or mom, but I'm posting this on the
list anyway.  Someday, someone will grab the damned thing and make
it part of groff.

Robert Goulding discovered that contents of the
system font/devps/download file must be present in
site-font/devps/download.  Similarly, freeeuro.pfa and EURO need
to be copied into site/font/devpdf/ and registered in its download
file.

I've updated the script so that it takes care of these details when
being run for the first time.

I'll also be amending the long-form font installation instructions
in the appendices to the momdocs.

-- 
Peter Schaffter
https://www.schaffter.ca