Add function getlocalbase() to libutil.

This function returns the path to the local software base directory, by
default "/usr/local" (or the value of _PATH_LOCALBASE in include/paths.h
when building the world).

The value returned can be overridden by 2 methods:

• the LOCALBASE environment variable (ignored by SUID programs)
• else a non-default user.localbase sysctl value

Reviewed by: hps (earlier version)
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D27236