Ensure that maxproc does not exceed pid_max, at the time of boot.
Changes to kern.pid_max mib after the boot can break this relation.

The maxfiles value was calculated by the MAXFILES formula based on
maxproc value, but this change decouples them, and MAXFILES now
references maxusers. Without manual tuning, the maxfiles default
value remains as it was prior to this commit. But for systems which
have tuned maxproc and rely on maxfiles to adjust, additional
reconfiguration is needed.

Reported by: rwatson
Reviewed by: emaste
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks