commit 378f8cb522 added these functions
(as stubs) but left them without declarations. this broke some
autoconf based software that detected linkability of the symbols but
didn't check for a declaration.
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.
this is needed in the long term for ABI compatibility anyway, and in
the immediate, it helps with building broken programs like GNU screen
that try to prototype the functions themselves rather than using the
header.
