Improve procstat check

[tuxillo]

Use found() method instead of checking for specific headers and symbols.

[tuxillo]

Not sure but I think it's a better way of checking for procstat availability.

[tuxillo]

any feedback? :)

[wulf7]

'procstat_open_sysctl' check was done to ensure that libprocstat can use sysctl kernel interface rather than kvm one. But as both interfaces was introduced with the same FreeBSD commit, this check is useless.
IMO we can drop libprocstat support entirely as it was outperformed by evdev sysctl interface. The only thing we should test in that case is webcamd+wayland combination. It may require including a user in to webcamd group to work.