Easier handling of 'CAPTCHA_DISABLED'.

The default will now be set in class_network.php if it's not in the database.

signup.php uses the config variable directly instead of querying the database.

See commit
1cb81f420eb1f503f98c07bc37e26d34f97e59c5
for when 'CAPTCHA_DISABLED' was intoduced.
Coding style.
Remove one leading tab.