Simplify kdb_cpw.c

Simplify kdb_cpw.c

In add_key_rnd, stop looking up the krbtgt DB entry; we have not used
it since 1.1.

Use copy_key_data in add_key_rnd and add_key_pwd.

krb5_dbe_crk, krb5_dbe_ark, krb5_dbe_def_cpw, and krb5_dbe_apw all
contained similar logic.  Consolidate all of them into a static helper
function which does the work of all four.  The ark/apw variants had
slightly different behavior then crk/cpw with keepold=true, so
introduce a three-value enum to express all three behaviors.