Use $(VALGRIND) when running programs using $(KRB5_RUN_ENV) or

Use $(VALGRIND) when running programs using $(KRB5_RUN_ENV) or
$(RUN_SETUP).  Replaces old hack with MAYBE_VALGRIND added to RUN_ENV
in a way that would break in some of the tests.

Set VALGRIND in site.exp in tests/dejagnu.  (Not used yet.)

Runs some shell scripts under valgrind, rather than changing them to
run only the executables under valgrind; this is mostly okay, just
creates lots of extra log data, and requires --trace-children=yes.

This should work for any instrumentation program invocation that gets
followed immediately by the name and argument list for the program
being instrumented.  For example, VALGRIND="env LD_PRELOAD=..." should
work, though I haven't tested it.

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@18699 dc483132-0cff-0310-8789-dd5450dbe970