Adds a cmake variable (DEBUG_FFMPEG_MUX) which enables FFmpeg debug
output in the ffmpeg-mux subprocess, and if on Windows, shows the
console window of the ffmpeg-mux subprocess so the current output can be
seen.
(Jim) printable_file allows the ability to keep a separate string for
logging which will not contain things like stream keys when used with
outputs such as HLS.
Instead of having ffmpeg-mux stored in a data directory, install it to
the primary binary directory. On windows, this fixes ffmpeg-mux
potentially accessing the wrong FFmpeg libraries (some programs install
them to system32, foolishly), and instead ensures that it uses the ones
that come with the program. On Linux, ensures that a binary is in its
appropriate directory (/usr/bin or /usr/local/bin rather than a subset
of /usr/share or /usr/local/share).
