msgexec: Add --newline option
See the commit 96dde0b for the rationale.
* gettext-tools/src/msgexec.c (newline): New variable.
(long_options): Add --newline option.
(main): Handle --newline option.
(usage): Document --newline option.
(process_string): Handle --newline option.
* gettext-tools/tests/msgexec-6: New file.
* gettext-tools/tests/Makefile.am (TESTS): Add new test.
* gettext-tools/doc/msgexec.texi: Document --newline option.
