As pointed out in the aftermath of CL 113997, splice is not supported
for SOCK_SEQPACKET or SOCK_DGRAM unix sockets. Don't call poll.Splice
in those cases.

Change-Id: Ieab18fb0ae706fdeb249e3f54d51a3292e3ead62
Reviewed-on: https://go-review.googlesource.com/136635
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>