Add support for Shell printf format strings.

Add support for Shell printf format strings.

* gettext-tools/src/message.h (enum format_type): Add format_sh_printf.
(NFORMATS): Increment.
* gettext-tools/src/message.c (format_language, format_language_pretty): Add an
entry for format_sh_printf.
* gettext-tools/src/format.h (formatstring_sh_printf): New declaration.
* gettext-tools/src/format.c (formatstring_parsers): Add an entry for
format_sh_printf.
* gettext-tools/src/format-sh-printf.c: New file, based on
gettext-tools/src/format-awk.c.
* gettext-tools/src/FILES: Mention it.
* gettext-tools/src/x-sh.h (SCANNERS_SH): Use formatstring_sh_printf as
secondary format string type.
* gettext-tools/src/xgettext.c (xgettext_record_flag): Update accordingly.
* gettext-tools/src/x-sh.c (init_flag_table_sh): Register gettext, ngettext with
flag 'pass-sh-printf-format'. Register 'printf' with flag 'sh-printf-format'.
* gettext-tools/src/Makefile.am (FORMAT_SOURCE): Add format-sh-printf.c.
* gettext-tools/libgettextpo/Makefile.am (libgettextpo_la_AUXSOURCES): Likewise.
* gettext-tools/doc/gettext.texi (PO Files): Mention sh-printf-format.
(sh-format): Document also the sh-printf-format strings.
* gettext-tools/doc/lang-sh.texi (sh): Mention the coreutils 'printf' command.
* gettext-tools/tests/xgettext-sh-1: Add a test case with a printf invocation.
* gettext-tools/tests/format-sh-printf-1: New file, based on
gettext-tools/tests/format-awk-1.
* gettext-tools/tests/format-sh-printf-2: New file, based on
gettext-tools/tests/format-awk-2.
* gettext-tools/tests/Makefile.am (TESTS): Add them.
* NEWS: Mention the change.