usr.bin/fortune: convert to OBJTOP/SRCTOP idioms

• Use OBJTOP/SRCTOP-relative paths when looking for include files and strfile.
• Add FORTUNES_OBJ and FORTUNES_SRC to abbreviate usr.bin/fortune pathing.

This is being done to simplify make output/idioms.

MFC after: 1 week
Reviewed by: bdrewery
Sponsored by: Dell EMC Isilon
Differential Revision: D9916