Build and install libafsauthent.so.2

Build and install libafsauthent.so.2

During the libtool interim, we had been building a .0 but not
installing it.  Prior to the libtoolization of shlibafsauthent, we
had installed a libafsauthent.so.1.1, which is the same version currently
installed by the 1.6 branch.  Since there have been backwards-incompatible
ABI changes (e.g., afsconf_BuildServerSecurityObjects) since the .1.1
version, we must bump the SONAME to .2.0.

At time of this writing, the libtool rules for updating the
version information is found at:
http://www.gnu.org/software/libtool/manual/html_node/Updating-version-info.html
and
http://www.gnu.org/software/libtool/manual/html_node/Libtool-versioning.html

This lets us consolidate the building of the shared and static libafsrpc
and their installation), as libtool will happily do both for us
at once.

We explicitly do not install the .la files, as our libtool use is
to be kept entirely internal.

Change-Id: I283f9bb74eb9853c268e8642ac1f01741deeae2b
Reviewed-on: http://gerrit.openafs.org/11462
Reviewed-by: D Brashear <shadow@your-file-system.com>
Tested-by: D Brashear <shadow@your-file-system.com>