Add macros to support testing of error categories to ldap.h

Modify ldap_result to assert returned error is not an one reserved
for API use.
Modify frontend LDAP operation routines to return an error code.
The returned value will be used to determine if an unsolicited notification
should be sent to the client.
Need to review returned error codes.  Namely some LDAP_PROTOCOL_ERROR
will like need to be changed (as they will cause unsolicited notifications).