It appears youtube-dl sometimes asks for a password on stdin. This won't
work, because mpv already uses the terminal.
(I wonder if this could be simpler, like simply closing FD 0, but let's
not. The FD would be reused by something random.)
If the stdout or stderr write callback is NULL, then don't redirect this
stream. Preparation for the next commit.
Not sure what to do on Windows; it seems STARTUPINFO doesn't allow
redirection only one of them. So just let them write nothing. For our
intended use-case (next commit), this is probably sensible.
The subprocess code was already split into fairly general functions,
separate from the Lua code. It's getting pretty big though, especially
the Windows-specific parts, so move it into its own files.
wm4
Kevin Mitchell
James Ross-Gowan