xgettext: In language YCP, avoid stack overflow.
* gettext-tools/src/x-ycp.c: Include error-progname.h.
(MAX_NESTING_DEPTH): New macro.
(nesting_depth): New variable.
(extract_parenthesized): Increase and check nesting_depth before calling
extract_parenthesized recursively.
(extract_ycp): Initialize nesting_depth.
* gettext-tools/tests/xgettext-ycp-stackovfl-1: New file.
* gettext-tools/tests/xgettext-ycp-stackovfl-2: New file.
* gettext-tools/tests/xgettext-ycp-stackovfl-3: New file.
* gettext-tools/tests/xgettext-ycp-stackovfl-4: New file.
* gettext-tools/tests/Makefile.am (TESTS): Add them.
