C#: Add support for dotnet.

C#: Add support for dotnet.

Reported by Michele Locati <mlocati@gmail.com>
at <https://savannah.gnu.org/bugs/index.php?66292>.

* DEPENDENCIES: Mention dotnet as an alternative to Mono.
* NEWS: Mention the change.
* PACKAGING: Mention the new file $prefix/lib/gettext/GNU.Gettext.dll.
* gettext-runtime/intl-csharp/Makefile.am (pkglibdir): New variable.
(CLEANFILES): Add GNU.Gettext.pdb.
(install-dll-no, install-dll-yes): Install also into $(pkglibdir).
(installdirs-dll, uninstall-dll): Update accordingly.
* gettext-runtime/intl-csharp/intl.cs (GettextResourceSet): Add field Table.
Override GetEnumerator, GetObject to handle it. Change the Keys method to handle
it as well.
* gettext-tools/tests/lang-csharp: In the program, on Windows, produce UTF-8
output with Unix end-of-lines and set the culture manually.