bsdinstall(8) hardening menu: Utilize new kern.randompid=1 behaviour

Enabling the PID randomization option in bsdinstall(8)'s hardening menu
now randomizes the effective value of kern.randompid on each boot.

Previous behaviour:
When kern.randompid was enabled via the the bsdinstall(8) hardening menu,
a random value was generated and placed in the systems /etc/sysctl.conf as
kern.randompid=value
This makes the value of kern.randompid static across reboots.

New behaviour:
When kern.randompid is enabled via the bsdinstall(8) hardening menu, the
line kern.randompid=1 is placed in the systems /etc/sysctl.conf.
This takes advantage of a new kernel feature and makes the value of
kern.randompid be randomized by the kernel on each reboot.

Submitted by: Marie Helene Kvello-Aune <marieheleneka@gmail.com>
Reviewed by: des
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D12433