Add connection pool support to rlm_krb5

This allows us  to remove the dependency on krb5_copy_context, and support threaded operation for more versions of MIT krb5
We need to set LDFLAGS and CFLAGS from SMART_CFLAGS and SMART_LDFLAGS when linking the krb5_thread_safe test program (else it might pick up the wrong version of the library)

We can now remove the dependency on MIT KRB5 > 1.11.4