Linux: create sockets with close-on-exec flag in thread-safe manner

Linux: create sockets with close-on-exec flag in thread-safe manner

There is a window of opportunity to leak file descriptors between
their creation and the fcntl(FD_CLOEXEC) call. If another thread forks
during this window, the descriptors will not have a the close-on-exec
flag, and get leaked after exec(). This is a limitation of POSIX.

While we're using the Linux-specific SOCK_CLOEXEC, we might as well
use SOCK_NONBLOCK, and spare ourselves the three fcntl() calls.