Auto merge of #1621 - coolreader18:fdset-redox-ai-android, r=gnzlbg

Add Redox FD_* function-macros and Android AI_* constants

I'm not sure if I'm allowed to add multiple, separate APIs in the same PR, but I guess they're both for uncommon/exotic platforms?

The `AI_*` constants were taken from `usr/include/netdb.h` on my Termux install; the `FD_` functions were taken from `linux_like/mod.rs`.