config::set should check for actual setting

[ju5t]

config::set checks if a setting exists in directadmin.conf. There are situations where this can be a problem. Especially when DirectAdmin changes some of their internal logic as happened with the SSL to ssl (upper to lowercase) transition.

We should use a similar approach as custombuild::config::set, but instead of checking the configuration, it should check the actual configuration like so:

./directadmin c | grep ^ssl=

and

./directadmin set ssl 1

This shouldn't be a major change.