readlink: support multiple command line arguments

readlink: support multiple command line arguments

This allows efficient processing of multiple files,
while also increasing compatibility with BSD's readlink(1).
We also add the -z, --zero option to delimit output items
with the NUL character which disambiguates output in the
presence of '\n' characters.

* src/readlink.c (usage): Add the --zero description,
and also adjust the description of --no-newline accordingly.
(main): Handle the -z option and iterate over multiple arguments.
Also as in commit v8.15-24-g9d46b25 we use fputs() and putchar()
rather than printf() for performance reasons.
* doc/coreutils.texi (readlink invocation): Document the
new --zero option, adjust the --no-newline description, and
tweak the general info to indicate multiple files are supported.
* tests/readlink/multi.sh: A new test for the new functionality.
* tests/local.mk: Reference the new test.
* man/readlink.x: Adjust the summary and also reference realpath.
* NEWS: Mention the improvement.
* THANKS.in: Suggested by Aaron Davies.