Replace literal uses of /usr/local with a variable

The variable defaults to "/usr/local", unless sysctl returns some other
value for "user.localbase".

The value of user.localbase defaults to _PATH_LOCALBASE as defined in
paths.h and thus this commit has no immediate effect.

The purpose of this change is to make /etc/defaults/rc.conf automatically
use the value of _PATH_LOCALBASE when not set to the default value.

Reviewed by: imp, scottl
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D27014