authX509: Make ldapusercert validation optional.

This patch makes it possible to skip validating the certificate
against an attribute in the LDAP directory, and instead trust that
Apache has checked it.

In addtition it contains some bugfixes and documentation fixes.

Thanks to Thijs Kinkhorst for this patch!

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@3234 44740490-163a-0410-bde0-09ae8108e29a