include: bump UV_VERSION_MINOR

Fixes #740.
unix: improve uv_guess_handle() implementation

Make it understand FIFOs, character devices and sockets.
stream: run try_select only for pipes and ttys

Its not necesary for TCP and other streams, since fd is always working
with kqueue there.
build: rename OS to PLATFORM

Rename the OS make variable to PLATFORM, it conflicts with the OS env
var. That is, running `make` when the OS env var is set, may cause
spurious build breakage.

Fixes #737.