[REVIEW] replace literal uses of /usr/local with a macro [D26942]

I have created

	https://reviews.freebsd.org/D26942

as a suggested patch to remove nearly 20 literal uses of /usr/local
in the base system.

This requires to add an include of paths.h to some of the source files
(.c or .h), but none of these includes is leaked to /usr/include and
they are thus only visible during the build.

I have built the world with this patch applied and the resulting
binaries are unchanged.

The definition of _PATH_LOCALBASE in paths.h could at a later time
be derived from the value of LOCALBASE (in src/Makefile.inc1 or
overridden my the user in src.conf), but this is a change that
should be discussed separately from this review.

Please comment on this patch, the decision to not touch contrib
sources and which follow-up steps to perform next (e.g. similar
changes to shell scripts or configuration files).