hyph: read hcode mappings from the third argument of .hpf

The .hpf and .hpfa requests read the hyphenation character mappings
from the address specified as their third arguments.  Now the three
arguments of these requests correspond to files whose names end
with .pat.txt, .hyp.txt and .chr.txt in
ftp://ftp.ctan.org/tex-archive/language/hyph-utf8/tex/generic/hyph-utf8/patterns/txt/