xgettext: Change the C# extractor to make use of mixed_string_buffer.
* gettext-tools/src/x-csharp.c (struct string_buffer): Remove type.
(init_string_buffer, string_buffer_append_unicode_grow,
string_buffer_append_unicode, string_buffer_result, free_string_buffer): Remove
functions.
(comment_buffer, comment_start, comment_at_start, comment_add,
comment_line_end): Use mixed_string_buffer API.
(phase6_get): Likewise.
