Add function getlocalbase() to libutil.

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