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 <[email protected]>
Closes openzfs#1058

zfs.spec.in

@@ -2,11 +2,8 @@
 %define version          @VERSION@
 %define release          @ZFS_META_RELEASE@
 %define debug_package    %{nil}
-%define _prefix          /
-%define _libexecdir      /usr/libexec
-%define _datadir         /usr/share
-%define _mandir          %{_datadir}/man
-%define _includedir      /usr/include
+%define _sbindir         /sbin
+%define _libdir          /%{_lib}
 %define _udevdir         /lib/udev
 
 Summary:         ZFS Library and Utils