mime: ParseMediaType returns os.Error now, not a nil map

mime: ParseMediaType returns os.Error now, not a nil map

ParseMediaType previously documented that it always returned
a non-nil map, but also documented that it returned a nil map
to signal an error.

That is confusing, contradictory and not Go-like.

Now it returns (mediatype string, params map, os.Error).

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/4867054