- now all write operations appear to work correctly with PostgeSQL 7.0
- all write operations have been made transactional (atomic writes to
  entries are committed separately only in case of complete^1 success
  while all other operations are rolled-back by default)
- more cleanup and handling of exceptional conditions

TODO:
- deen to check with different databases and more up to date versions
  of both unixODBC and PostgreSQL.

^1: attribute add/modify/delete operations silently succeed if the
    appropriate add/delete proc does not exist for each attribute;
    this may be correct to hide undesired/unimplemented correspondence
    between LDAP and SQL databases; however, a more appropriate
    LDAP behavior would be a failure with LDAP_UNAVAILABLE if a
    single write operation cannot be executed for such reason