Add a DisallowUnknownFields flag to Decoder.

DisallowUnknownFields causes the Decoder to return an error when
the the decoding destination is a struct and the input contains
object keys which do not match any non-ignored, public field the
destination, including keys whose value is set to null.

Note: this fix has already been worked on in 27231, which seems
to be abandoned. This version is a slightly simpler implementation
and is up to date with the master branch.

Fixes #15314

Change-Id: I987a5857c52018df334f4d1a2360649c44a7175d
Reviewed-on: https://go-review.googlesource.com/74830Reviewed-by: Joe Tsai <joetsai@google.com>
Run-TryBot: Joe Tsai <joetsai@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>