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.

tests/readlink/multi.sh

@@ -0,0 +1,46 @@
+#!/bin/sh
+# test multiple argument handling.
+
+# Copyright (C) 2012 Free Software Foundation, Inc.
+
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+. "${srcdir=.}/tests/init.sh"; path_prepend_ ./src
+print_ver_ readlink
+
+touch regfile || framework_failure_
+ln -s regfile link1 || framework_failure_
+
+readlink link1 link1 || fail=1
+readlink link1 link2 && fail=1
+readlink link1 link2 link1 && fail=1
+readlink -m link1 link2 || fail=1
+
+printf '/1\0/1\0' > exp || framework_failure_
+readlink -m --zero /1 /1 > out || fail=1
+compare exp out || fail=1
+
+# The largely redundant --no-newline option is ignored with multiple args.
+# Note BSD's readlink suppresses all delimiters, even with multiple args,
+# but that functionality was not thought useful.
+readlink -n -m --zero /1 /1 > out || fail=1
+compare exp out || fail=1
+
+# Note the edge case that the last xargs run may not have a delimiter
+rm out || framework_failure_
+printf '/1\0/1\0/1' > exp || framework_failure_
+printf '/1 /1 /1' | xargs -n2 readlink -n -m --zero >> out || fail=1
+compare exp out || fail=1
+
+Exit $fail