Simplify libkadm5 client realm initialization

Simplify libkadm5 client realm initialization

The "realm" variable in init_any is used only to fill in the realm of
the service principal in gic_iter().  The service principal realm
should always be the realm we looked up config parameters for, so we
can supply that realm to get_init_creds() unconditionally and
eliminate the case where we use the client principal realm.

Also get rid of an outdated comment and an #if 0 block we will never
need again, and use SNPRINTF_OVERFLOW to check the snprintf result.