Modernize some LDAP sources

Modernize some LDAP sources

Bring ldap_misc.c up to date with current practices and make limited
changes to other files.  Of note:

* krb5_decode_krbsecretkey was freeing its bvalues argument; make that
  the caller's responsibility.
* Make is_principal_in_realm and has_modify_increment return
  krb5_boolean, reversing the sense of their results.
* Remove broken code path in decode_tl_data when an integer value has
  a length other than 2 (which should never happen).
* Simplify krb5_ldap_readpassword and make it take filename/name
  parameters instead of an LDAP context.
* Make krb5_ldap_bind (renamed to authenticate) responsible for
  setting a useful error message, so that its caller doesn't assume
  knowledge of the bind parameters.
* Make krb5_ldap_initialize (renamed to initialize_server) responsible
  for updating the handle list, and remove the otherwise unused
  krb5_update_ldap_handle.
* Remove remaining skeletal certificate support, including the unused
  has_sasl_external_mech function.
* Remove unused krb5_get_containerdn and KDB_TL_CONTAINERDN.
* Remove kdb_xdr.h; all of its prototypes were for functions that
  don't exist in the module or were duplicated in other headers.
* Remove krb5_ldap_get_strings and use ldap_get_values directly at
  its call sites; there was no need to copy the result.