Don't require krb5.conf without KRB5_DNS_LOOKUP

Don't require krb5.conf without KRB5_DNS_LOOKUP

For a long time we have allowed krb5 contexts to be initialized in the
absence of krb5.conf--but only if KRB5_DNS_LOOKUP is defined,
presumably on the theory that no KDCs could be contacted without
either DNS support or profile configuration.  But locate plugins could
provide the ability to find KDCs, and some libkrb5 operations (such as
IAKERB initiation) could succeed without needing to locate KDCs.

Also get rid of the profile_in_memory context flag, since we don't use
it any more.