The code to lookup a value in DEBUGFILES usually operates on a file
name, but there are few non-file uses. The latter wouldn't work on
VMS because of the way it was manipulating the name: first stripping
away path, suffix, and version, then adding hardcoded ".c" suffix on.
I thought we already had a routine to get the base part of a name
from a full path, but if so, I haven't been able to find it. This
adds new nh_basename() to do that, with the option of either keeping
or discarding the suffix or type portion.
The VMS usage that prompted this hasn't actually been tested.
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