If the user tries to set kern.randompid to 1 (which is meaningless), set
it to a random value between 100 and 1123, rather than 0 as before.

Submitted by: Marie Helene Kvello-Aune <marieheleneka@gmail.com>
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D5336