Update PHP API change gor ldapwhoami (#674)

I have been maintaining the PHP LDAP EXOP patch for a few years,
which include the ldapwhoami() function. This has finally made its
way into PHP distribution and will be available in PHP 7.3, but
with a modified prototype.

This changes adapts to this API change. While there, also update
exception handling on par with recent SimpleSAMLphp code.