net: add Buffers type, do writev on unix

No fast path currently for solaris, windows, nacl, plan9.

Fixes #13451

Change-Id: I24b3233a2e3a57fc6445e276a5c0d7b097884007
Reviewed-on: https://go-review.googlesource.com/29951
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
diff --git a/src/syscall/syscall_nacl.go b/src/syscall/syscall_nacl.go
index 88e4e3a..3247505 100644
--- a/src/syscall/syscall_nacl.go
+++ b/src/syscall/syscall_nacl.go
@@ -296,3 +296,5 @@
 func ParseRoutingMessage(b []byte) ([]RoutingMessage, error)      { return nil, ENOSYS }
 func ParseRoutingSockaddr(msg RoutingMessage) ([]Sockaddr, error) { return nil, ENOSYS }
 func SysctlUint32(name string) (value uint32, err error)          { return 0, ENOSYS }
+
+type Iovec struct{} // dummy