eExperiments.php add experiment ethnicity options

The placeholder values have been replaced with the same ethnicity
options on the login page.
pEnroll.php added validate_user_against_requir...

Added function validate_user_against_requirement($expid, $username);

To use, provide it with an expid and a username and if that user meets
the requirements for that experiment the function will return true and
false otherwise. This is TOTALLY UNTESTED because I've not got internet
access at present.