split: add the --additional-suffix option

Add the --additional-suffix option, to append an
additional static suffix to output file names.

* src/split.c (next_file_name): Append suffix to output file names.
(main): Handle new --additional-suffix option.
* NEWS (New features): Mention it.
* doc/coreutils.texi (split invocation): Mention it.
* tests/split/additional-suffix: New file. Test --additional-suffix.
* tests/Makefile.am (TESTS): Add it.
Requested by Peng Yu, in bug 6554

tests/split/additional-suffix

@@ -0,0 +1,44 @@
+#!/bin/sh
+# show that 'split --additional-suffix=SUFFIX' works.
+
+# 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=.}/init.sh"; path_prepend_ ../src
+print_ver_ split
+
+printf '1\n2\n3\n4\n5\n' > in || framework_failure_
+
+split --lines=2 --additional-suffix=.txt in > out || fail=1
+cat <<\EOF > exp-1
+1
+2
+EOF
+cat <<\EOF > exp-2
+3
+4
+EOF
+cat <<\EOF > exp-3
+5
+EOF
+
+compare exp-1 xaa.txt || fail=1
+compare exp-2 xab.txt || fail=1
+compare exp-3 xac.txt || fail=1
+
+# Additional suffix must not contain slash
+split --lines=2 --additional-suffix=a/b in 2>/dev/null > out && fail=1
+
+Exit $fail