Allow reading the following sysctl MIBs in capability mode:
kern.hostname, kern.domainname, and kern.hostuuid

This allows sandboxed applications to read these sysctls

Submitted by: cem (original version)
Reviewed by: cem, jonathan, rwatson (original version)
Sponsored by: ScaleEngine Inc.
Differential Revision: https://reviews.freebsd.org/D8015