Update to require sanlock 1.8 for license compliance

Update to require sanlock 1.8 for license compliance

Inexplicably the sanlock code all got placed under the GPLv2-only,
so libvirt's use of sanlock introduces a license incompatibility.
The sanlock developers have now rearranged the code such that there
is a 'sanlock_client.so' which is LGPLv2+ while their daemon remains
GPLv2-only. To use the new client library we need to call the new
sanlock_init and sanlock_align APIs instead of sanlock_direct_init
and sanlock_direct_align. These APIs calls are now routed via the
sanlock daemon, instead of doing direct I/O calls to disk.

For all this we require sanlock >= 1.8

* configure.ac: Check for sanlock_client.so instead of sanlock.so
  and fix various comments
* libvirt.spec.in: Mandate sanlock >= 1.8
* src/Makefile.am: Link to -lsanlock_client
* src/locking/lock_driver_sanlock.c: Use sanlock_init and
  sanlock_align