procstat(1): add ability to specify subcommands not requiring pid lists

Add PS_MODE_NO_KINFO_PROC cmd modifier that indicates that neither
process list should be queried from the kernel, nor list of pids or -a
switch provided on the command line to filter the output.

This is intended for use by commands that query information not
neccessary attributed to specific process.

Reviewed by: markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D34834