net: optimize WriteToUDPAddrPort

See the recent change optimizing WriteMsgUDPAddrPort
for an explanation of why this change includes copy/paste/modify
instead of implementing WriteToUDP using WriteToUDPAddrPort.

name                          old time/op    new time/op    delta
WriteToReadFromUDPAddrPort-8    5.02µs ± 3%    4.71µs ± 2%   -6.31%  (p=0.000 n=15+14)

name                          old alloc/op   new alloc/op   delta
WriteToReadFromUDPAddrPort-8     68.0B ± 0%      4.0B ± 0%  -94.12%  (p=0.000 n=15+15)

name                          old allocs/op  new allocs/op  delta
WriteToReadFromUDPAddrPort-8      3.00 ± 0%      1.00 ± 0%  -66.67%  (p=0.000 n=15+15)

Change-Id: I301715e774de07eb6ccb4e329ccf2e554609abc1
Reviewed-on: https://go-review.googlesource.com/c/go/+/360599


Trust: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>