This is an attempt at making the interaction between
these three constructs clearer. Specifically:

- return statements terminate a function, execute deferred
  functions, return to the caller, and then execution
  continues after the call

- panic calls terminate a function, execute deferred
  functions, return to the caller, and then re-panic

- deferred functions are executed before a function _returns_
  to its caller

The hope is that with this change it becomes clear when a
deferred function is executed (when a function returns),
and when it is not (when a program exits).

R=r, rsc, iant, ken, iant
CC=golang-dev
https://golang.org/cl/6736071