add android/oboe notes

add a liboboe audio backend for android Oboe is a Google C++ library for audio on Android. The backend is currently based on the libao skeleton since I couldn't get it to play audio in a non-blocking way. (It would also be pointless since there isn't a way to poll(1).) It would be worthing experimenting with the callback API. So far, it works on my phone under termux. I can control amused with amused-web. It still lacks a test in the configure since we would need to use C++ for it.

add compat for getdtablesize()

fix build when there is no system endian.h

note that libmd is needed on mac too

ooops, fix path

add missing #include for endian.h in imsg compat

work around missing endian.h As usual, if not found try sys/endian.h or pull in the macos thingy.

(cont) work around lack of SOCK_{CLOEXEC,NONBLOCK} funny how they're trying to deprecate daemon() in favour of the mess of posix_spawn and still don't provide genuinely useful stuff like SOCK_CLOEXEC, NONBLOCK and pipe2(2).

work around missing SOCK_{CLOEXEC,NONBLOCK} on some dumb OSes

work around (a possibly missing) pipe2(2) Mac OS X... sigh...

mention libao in the README

add an libao audio backend This backend uses libao to play music. It's a bit convoluted since libao doesn't provide an async interface, so we have to run it in a separate thread. Then, there's some notification via a shared socketpair because the main loop is around poll(). This actually doesn't work OOTB on OpenBSD due to a restrictive pledge(), but it's not a issue since sndio should be used there. libao is the last resort.

trailing whitespace

amused-web: typo in error message