- Automatically determine the first argument to check.
- Skip checking matching non-variadic functions.
- Skip checking matching functions accepting non-interface{}
  variadic arguments.
- Removed fragile 'magic' code for special cases such as math.Log
  and error interface.

Fixes #15067
Fixes #15099

Change-Id: Ib313557f18b12b36daa493f4b02c598b9503b55b
Reviewed-on: https://go-review.googlesource.com/21513
Run-TryBot: Rob Pike <r@golang.org>
Reviewed-by: Rob Pike <r@golang.org>