NHDT substitution version 2.

Re-run nhgitset.pl to install.
"perldoc DEVEL/hooksdir/nhsub" for details.  General docs still to come.
Quick notes:
- "git nhsub" lets you apply substitutions to a file without involving any
  version control.
- When doing nhadd/nhcommit, the working directory WILL reflect the results
  of the substitutions.
Let's see what this breaks.

.gitattributes

@@ -1,5 +1,5 @@
-*.[ch] filter=NHtext merge=NHsubst
-*.sh filter=NHtext merge=NHsubst
+*.[ch] NHSUBST
+*.sh NHSUBST
 * text=auto
 *.hqx -text
 *.sln -text