allow a custom error log file for plugins by means of a slapd.conf directive; add very bare-bone back-monitor info about installed plugins

improve limits handling and consistency; return "Admin limit exceeded" instead of "Unwilling to perform"

(already removed from usage statements)

with each password-hash scheme.

Now related ITSes need be audited and possibly closed.

Enhancements:
  - re-styled code for better readability
  - upgraded backend API to reflect recent changes
  - LDAP schema is checked when loading SQL/LDAP mapping
  - AttributeDescription/ObjectClass pointers used for more efficient
    mapping lookup
  - bervals used where string length is required often
  - atomized write operations by committing at the end of each operation
    and defaulting connection closure to rollback
  - added LDAP access control to write operations
  - fully implemented modrdn (with rdn attrs change, deleteoldrdn,
    access check, parent/children check and more)
  - added parent access control, children control to delete operation
  - added structuralObjectClass operational attribute check and
    value return on search
  - added hasSubordinate operational attribute on demand
  - search limits are appropriately enforced
  - function backsql_strcat() has been made more efficient
  - concat function has been made configurable by means of a pattern
  - added config switches:
      - fail_if_no_mapping	write operations fail if there is no mapping
      - has_ldapinfo_dn_ru	overrides autodetect
      - concat_pattern		a string containing two '?' is used
				(note that "?||?" should be more portable
				than builtin function "CONCAT(?,?)")
      - strcast_func		cast of string constants in "SELECT DISTINCT					statements (needed by PostgreSQL)
      - upper_needs_cast	cast the argument of upper when required
				(basically when building dn substring queries)

Todo:
  - add security checks for SQL statements that can be injected (?)
  - re-test with previously supported RDBMs
  - replace dn_ru and so with normalized dn (no need for upper() and so
    in dn match)
  - implement a backsql_normalize() function to replace the upper()
    conversion routines
  - note that subtree deletion, subtree renaming and so could be easily
    implemented (rollback and consistency checks are available :)
  - implement "lastmod" and other operational stuff (ldap_entries table ?)