pkgbase: split kerberos binaries and libs

Summary:
This allows installing packages that depend on kerberos libraries
without pulling in all the binaries. It also moves libgssapi to runtime
to allow installing kerbereos libraries without adding a dependancy on
the large utilities package. It makes sense to put libgssapi in runtime
rather than kerberos-lib since this is a plugin layer which is intended
to support any GSS-API mechanisms, not just kerberos.

A good example of a package which uses kerberos libraries without
needing the kerberos utilities is sshd. This uses the kerberos GSS-API
libraries to implement its GSSAPIAuthentication option.

MFC after: 2 weeks

Subscribers: imp

Differential Revision: https://reviews.freebsd.org/D36028

(cherry picked from commit 6e4dbb7feff42275c3025dbc25d6ea6cce4e5fd7)