Add option to build without libkeyutils

Add option to build without libkeyutils

Add a --without-keyutils option to disable the keyring ccache, for
scenarios where minimizing libkrb5 dependencies is important.

[ghudson@mit.edu: moved processing to configure.ac; added error if
explicit --without-keyutils is given and libkeyutils is not found;
nested persistent keyring check for clarity; moved new documentation
to appropriate place and don't mention persistent keyrings; rephrased
commit message]

ticket: 8791 (new)