Refactor socket.tcp_connect() function

This patch fixes logic of non-blocking tcp connect as it
described in man pages and Unix Network Programming vol.1.
Set default value for socket:shutdown(how) argument to SHUT_RDWR
Merge branch 'stable'

Conflicts:
	src/lua/bsdsocket.lua
	test/box/bsdsocket.result
	test/box/bsdsocket.test