Make LOCALBASE available as a sysctl value

[se]

Summary

Make the value of _PATH_LOCALBASE the base system was built with available to user-land programs, libraries and scripts

This sysctl variable could e.g. be used in the rc framework to adjust values in /etc/defaults/rc.conf to the LOCALBASE of the running system, in most cases removing the need to locally overwrite the default values.
Start-up scripts that have to consider port/package supplied data could also be adjusted to use this value.

Test Plan

Build kernel and world with these patches applied
Test that "sysctl user.localbase" returns the value that has been used to build the world.