pf: Make request_maxcount runtime adjustable

[kp]

Summary

There's no reason for this to be a tunable. It's perfectly safe to
change this at runtime.

[donner]

What happens, if the number is reduced below the number of currently existing tables?

[kp]

This affects the number of items (not just tables) that can be processed in a single ioctl() operation, not the number of entries we allow in a table.
pfctl by and large only works on a single table at a time, but might try to add/delete/modify/... multiple IP addresses at a time. It's generally possible to do this in multiple requests, so having more items in a table than the request_maxcount isn't a critical problem.