(ITS#5663) Declaring substitute syntax as LDAP syntax

[michael@stroeder.com]

Full_Name: Michael Ströder
Version: HEAD
OS: 
URL: ftp://ftp.openldap.org/incoming/
Submission from: (NULL) (84.163.84.234)


Sometimes one wants to add a custom schema which also defines LDAP syntaxes
which are not supported yet but could be substituted by another LDAP syntax
already implemented.

Instead of changing SYNTAX of all attribute type declarations in the custom
schema it might be handy if declaring a substitute LDAP syntax would be possible
while still providing the original syntax declaration to the client which might
know better what to do with it.

Example for emulating custom LDAP syntax with PrintableString:
ldapsyntax ( 1.3.6.1.4.1.5427.666.42.42.42 X-SUBST 1.3.6.1.4.1.1466.115.121.1.44
)

This should fail with parsing error if one trys to add a substitute syntax for a
syntax already implemented.