a research in debian codesearch and grepping over the pkgsrc
directory tree have shown that these macros are all either unused,
or defined by programs in case they need them.

the pathnames prefixed with /dev/null/ are guaranteed never to be
valid. the previous use of /dev/null alone was mildly dangerous in
that bad software might attempt to unlink the name when it found a
non-regular file there and create a new file.

one file was reusing another file's macro name, and many had
inconsistent underscores and application of SYS prefix, etc.
patch by Szabolcs Nagy (nsz)