xgettext: Recognize 'env' and assignments in Shell scripts.

xgettext: Recognize 'env' and assignments in Shell scripts.

Reported by Jason Vas Dias <jason.vas.dias@gmail.com>
in <https://lists.gnu.org/archive/html/bug-gettext/2019-08/msg00032.html>.

* gettext-tools/src/x-sh.c (enum word_type): Add t_assignment.
(read_word): Recognize assignments.
(read_command): In a function position, ignore assignments and 'env' tokens.
* gettext-tools/tests/xgettext-sh-7: New file.
* gettext-tools/tests/Makefile.am (TESTS): Add it.
* NEWS: Mention the change.