c_client.py: Always open output files in UTF-8
The UTF-8 quotes can appear in man pages etc as well, not only C code. For example, in xcb_composite_redirect_subwindows.3:
xcb_composite_redirect_subwindows - Redirect all current and future
children of ‘window’
Note that window is quoted by a pair of UTF-8 single quotes.
Closes: #72