linux: fix setsockopt(SO_REUSEPORT) error handling

Linux as of 3.9 has a SO_REUSEPORT option that is similar but not
identical to its BSD counterpart.

On the BSDs, it turns on SO_REUSEADDR _and_ makes it possible to share
the address and port across processes.

On Linux, it "merely" enables fair load distribution - port sharing
still requires that you set SO_REUSEADDR.

Fair distribution is a desirable trait but not an essential one.
We don't know in advance whether the kernel actually supports
SO_REUSEPORT so don't treat EINVAL or ENOPROTOOPT as errors.

As an aside, on the BSDs we now omit the setsockopt(SO_REUSEADDR)
system call because it's implied by SO_REUSEPORT.

Fixes #870.
linux: don't turn on SO_REUSEPORT socket option

On the BSDs, SO_REUSEPORT is pretty much SO_REUSEADDR with some special
casing for IP multicast.  When two processes (that don't do multicast)
bind to the same address, only the last one receives traffic.  It allows
one to "steal" the bound address from another process.  (Both processes
have to enable SO_REUSEPORT though, so it only works in a cooperative
setting.)

On Linux however, it enables port sharing, not stealing - both processes
receive a share of the traffic.  This is a desirable trait but pre-3.9
kernels don't support the socket option and a libuv program therefore
behaves differently with older kernels or on another platform.

The difference in behavior (sharing vs stealing) is, in my opinion,
bigh and confusing enough that it merits a rollback.

This commit effectively reverts commit 17452cd.