Replace literal uses of /usr/local in C sources with _PATH_LOCALBASE

Literal references to /usr/local exist in a large number of files in
the FreeBSD base system. Many are in contributed software, in configuration
files, or in the documentation, but 19 uses have been identified in C
source files or headers outside the contrib and sys/contrib directories.

This commit makes it possible to set _PATH_LOCALBASE in paths.h to use
a different prefix for locally installed software.

In order to avoid changes to openssh source files, LOCALBASE is passed to
the build via Makefiles under src/secure. While _PATH_LOCALBASE could have
been used here, there is precedent in the construction of the path used to
a xauth program which depends on the LOCALBASE value passed on the compiler
command line to select a non-default directory.

This could be changed in a later commit to make the openssh build
consistently use _PATH_LOCALBASE. It is considered out-of-scope for this
commit.

Reviewed by: imp
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D26942