internal/poll: fall back on unsupported splice from unix socket

Gracefully fallback to a userspace copy when the kernel does not support
splice(2) on a unix domain socket. EINVAL is returned by the splice
syscall if it does not support unix domain sockets. Keeping the handled
return value as false when the first splice call fails with EINVAL will
cause the caller to fall back to a userspace copy.

Fixes #27513

Change-Id: I4b10c1900ba3c096cb32edb7c8a6044f468efb52
Reviewed-on: https://go-review.googlesource.com/133575


Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>