Improve handling of non-BMP characters with Tcl 9.0.
* gettext-tools/src/write-tcl.c (hexdigit): New variable.
(write_tcl8_string): Renamed from write_tcl_string. Enable the escaping of
braces.
(write_tcl9_string, is_entirely_ucs2, write_tcl_string): New functions.
* gettext-tools/tests/msgfmt-tcl-1: New file.
* gettext-tools/tests/testdata/tcltest_pl.po: New file.
* gettext-tools/tests/testdata/tcltest_pl.msg: New file.
* gettext-tools/tests/msgfmt-tcl-2: New file.
* gettext-tools/tests/testdata/tcltest_cs.po: New file.
* gettext-tools/tests/testdata/tcltest_cs.msg: New file.
* gettext-tools/tests/Makefile.am (TESTS): Add msgfmt-tcl-1, msgfmt-tcl-2.
(EXTRA_DIST): Add the new testdata files.
* NEWS: Mention the change.
