Add search of LOCALBASE/share/calendar for calendars supplied by a port.

Calendar files in LOCALBASE override similarily named ones in the base
system. This could easily be changed if the base system calendars should
have precedence, but it could lead to a violation of POLA since then the
port's files were ignored unless those in base have been deleted.

There was no definition of _PATH_LOCALBASE in paths.h, but verbatim uses
of /usr/local existed for _PATH_DEFPATH. Use _PATH_LOCALBASE here to ease
a consistent modification of this prefix.

Reviewed by: imp, pfg
Differential Revision: https://reviews.freebsd.org/D26882