[conformal/btcd] 1184a4: Prevent hang on shutdown race.

[Dave Collins]

  Branch: refs/heads/master
  Home:   https://github.com/conformal/btcd
  Commit: 1184a4d8d1a0dbeab6bc113777bb7b72af2574f2
      
https://github.com/conformal/btcd/commit/1184a4d8d1a0dbeab6bc113777bb7b72af2574f2
  Author: Dave Collins <da...@conformal.com>
  Date:   2014-07-22 (Tue, 22 Jul 2014)

  Changed paths:
    M signal.go

  Log Message:
  -----------
  Prevent hang on shutdown race.

This commit prevents a race that could happen on shutdown if a sigint was
received between adding the first the interrupt handler and and future
handlers.  This code is loosely based on initial pull request #154 which
was not accepted due to introducing a race of its own.

Thanks to @tuxcanfly for the intial pull request and finding the issue.