xgettext: In language C#, avoid stack overflow.

xgettext: In language C#, avoid stack overflow.

* gettext-tools/src/x-csharp.c (MAX_NESTING_DEPTH): New macro.
(paren_nesting_depth, brace_nesting_depth): New variables.
(extract_parenthesized): Increase and check paren_nesting_depth or
brace_nesting_depth before calling extract_parenthesized recursively.
(extract_csharp): Initialize paren_nesting_depth and brace_nesting_depth.
* gettext-tools/tests/xgettext-csharp-stackovfl-1: New file.
* gettext-tools/tests/xgettext-csharp-stackovfl-2: New file.
* gettext-tools/tests/xgettext-csharp-stackovfl-3: New file.
* gettext-tools/tests/xgettext-csharp-stackovfl-4: New file.
* gettext-tools/tests/Makefile.am (TESTS): Add them.