eldap: Add support for modifying passwords

This implements the LDAP Password Modify Extended Operation (RFC 3062)
with two new functions modify_password/3 and modify_password/4. The
former is for directly setting passwords, and the latter is for users to
change 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).