Add env and detach arguments. This means the command.c code must use the
"new" mp_subprocess2(). So also take this as an opportunity to clean up.
win32 support gets broken by it, because it never made the switch to the
newer function.
The new detach parameter makes the "run" command fully redundant, but I
guess we'll keep it for simplicity. But change its implementation to use
mp_subprocess2() (couldn't do this earlier, because win32).
Privately, I'm going to use the "env" argument to add a key binding that
starts a shell with a FILE environment variable set to the currently
playing file, so this is very useful to me.
Note: breaks windows, so for example youtube-dl on windows will not work
anymore. mp_subprocess2() has to be implemented. The old functions are
gone, and subprocess-win.c is not built anymore. It will probably work
on Cygwin.
In a bunch of cases, we emulate highly platform specific APIs on a
higher level across all OSes, such as IPC, terminal, subprocess
handling, and more. We have source files for each OS, and they implement
all the same mpv internal API.
Selecting which source file to use on an OS can be tricky, because there
is partially overlapping and emulated APIs (consider Cygwin on Windows).
Add a pick_first_matching_dep() function to make this slightly easier
and more structured.
Also add dummy backends in some cases, to deal with APIs not being
available.
Clarify the Windows dependency identifiers, as these are the most
confusing.
wm4