Fix broken RPATH in spec file

Fix broken RPATH in spec file

Rather then setting _prefix=/ and having to override all the
default install locations.  It's cleaner, and more understandable,
to leave prefix=/usr and only override _sbindir and _libdir.  This
fixes three issues:

* The commands no longer get built with an incorrect rpath for
  the libraries.   This is good because fixing this sort of
  thing is required by the Fedora packaging guidelines.

    http://fedoraproject.org/wiki/Packaging:Guidelines#Beware_of_Rpath

* The various AUTHORS, COPYRIGHT, etc files are now correctly
  installed under /usr/share/doc instead of /share/doc.

* _libexecdir is now handled properly for each distribution.
  Fedora/RHEL=/usr/libexec, OpenSUSE/SLES=/usr/lib, Debian=/usr/lib/rpm

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1058