xgettext: In language Shell, avoid stack overflow.

xgettext: In language Shell, avoid stack overflow.

* gettext-tools/src/x-sh.c (MAX_NESTING_DEPTH): New macro.
(nesting_depth): New variable.
(read_word): Increase nesting_depth before calling read_command_list
recursively.
(read_command_list): Check nesting_depth.
(extract_sh): Initialize nesting_depth.
* gettext-tools/tests/xgettext-shell-stackovfl-1: New file.
* gettext-tools/tests/xgettext-shell-stackovfl-2: New file.
* gettext-tools/tests/xgettext-shell-stackovfl-3: New file.
* gettext-tools/tests/xgettext-shell-stackovfl-4: New file.
* gettext-tools/tests/xgettext-shell-stackovfl-5: New file.
* gettext-tools/tests/Makefile.am (TESTS): Add them.