kern: Introduce kern.pid_max_limit sysctl.

The kern.pid_max_limit will hold the PID_MAX value the kernel was
compiled with. The existing kern.pid_max sysctl can be modified and
doesn't really represent maximum PID number in the system, as there
may still be processes created with higher PIDs before kern.pid_max
was lowered.

Reivewed by: kib, zlei
Approved by: oshogbo
Differential Revision: https://reviews.freebsd.org/D43077