eldap: Add support for LDAP Password Modify Extended Operation (RFC 3062)

This adds to new functions, modify_password/3 and modify_password/4, to support password modification requests.  The former is for direct setting of passwords, and the latter is for users changing their own passwords.

Since all three parameters are optional, I've opted to support this with the empty string rather than a property list.  This seems consistent with other functions in the module (i.e. modify_dn/5), but is also rather awkward to use. Any alternative suggestions?