Socket descriptors are not closed when fd.connect() fails during generic socket creation.
After a connection failure [ECONNREFUSED] descriptors are left in SYN_SENT state indefinitely (unless they get an explicit RST). Repeated failed connections will eventually cause your program to hit the user/system max-open-files limit.

Fixes #2349.

R=golang-dev, mikioh.mikioh
CC=golang-dev
https://golang.org/cl/5229047