pfctl: Point users to net.pf.request_maxcount if large requests are rejected

The kernel will reject very large tables to avoid resource exhaustion
attacks. Some users run into this limit with legitimate table
configurations.

The error message in this case was not very clear:

pf.conf:1: cannot define table nets: Invalid argument
pfctl: Syntax error in config file: pf rules not loaded

If a table definition fails we now check the request_maxcount sysctl,
and if we've tried to create more than that point the user at
net.pf.request_maxcount:

pf.conf:1: cannot define table nets: too many elements.
Consider increasing net.pf.request_maxcount.
pfctl: Syntax error in config file: pf rules not loaded

PR: 235076
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D18909