nfs-utils: fix a number of specfile problems
    
    The package review for RHEL7 turned up a number of problems with the
    current nfs-utils specfile. This patch aims to fix most of them:
    
    - Swap out hardcoded paths for macros that are defined for this purpose
    - Move the systemd preconfig/postconfig scripts to /usr/libexec
    - Fix URL for Source0
    - add directives to clean out $RPM_BUILD_ROOT in %install and %clean
    - add a %defattr() directive to ensure that all files are owned by root
    - get rid bogus %attr() settings on symlinks
    
    Signed-off-by: Jeff Layton <jlayton@redhat.com>
    Signed-off-by: Steve Dickson <steved@redhat.com>