This changes the misc/dist program to generate OS X
packages using pkgbuild and productbuild.

The productbuild utility makes it easy to generate
packages with a custom Distribution file.  This allows
us to add an installcheck script that presents a
friendly message to users who are running on an old
version of Mac OS X.

The change also fixes a few issues with the
postinstall script:

 - In-repo version of the script has been made
   executable. Installers generated using the new
   tools couldn't execute it otherwise.

 - It now uses -d for checking for the existence
   of the Xcode specs directory.

 - The call to sudo.bash has been dropped since cov
   and prof aren't bundled with the binary
   distributions.

Fixes #3455.

Tested on 10.5.8, 10.6.0, 10.6.8 and 10.7.3.

R=adg, golang-dev
CC=golang-dev
https://golang.org/cl/5987044