We want to distinguish actual errors, and just aborting the program
intentionally.
Also be a bit more careful with handling the wait() exit status: do not
called WEXITSTATUS() without checking WIFEXITED() first.
It's useless, and creates a bogus warning in subprocess-posix.c.
Since I don't know which compilers might have it by default, just change
it to -Wno-redundant-decls.
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.
