This is just a more convenient way to start IPC client scripts per mpv
instance.
Does not work on Windows, although it could if the subprocess and IPC
parts are implemented (and I guess .exe/.bat suffixes are required).
Also untested whether it builds on Windows. A lot of other things are
untested too, so don't complain.
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.
