os/exec: document that Stdin goroutine must finish in Wait

Fixes issue 7990.

LGTM=iant, bradfitz
R=bradfitz, iant, robryk
CC=golang-codereviews
https://codereview.appspot.com/156220043
database/sql: add Drivers, returning list of registered drivers

Fixes issue 7969.

LGTM=bradfitz
R=bradfitz
CC=golang-codereviews
https://codereview.appspot.com/158950043
runtime: remove hand-generated ptr bitmaps for reflectcall

A Go prototype can be used instead now, and the compiler
will do a better job than we will doing it by hand.
(We got it wrong in amd64p32, causing the current build
breakage.)

The auto-prototype-matching only applies to functions
without an explicit package path, so the TEXT lines for
reflectcall and callXX are s/runtime·/·/.

LGTM=khr
R=khr
CC=golang-codereviews, iant, r
https://codereview.appspot.com/153600043