makedb: Fix apropos database generation better across release/
The apropos database format used by our new man(1) is different and
incompatible to that used by our old man(1). The files are also named
differently, mandoc.db (new) and whatis (old).
So it makes no sense to use the old makewhatis on new systems or the
new makewhatis on old systems. If the desired makewhatis does not
exist, then we just don't generate the db, because the building system
doesn't have the makewhatis needed to generate it.
Once installed, the database will be updated regularly as per weekly
periodic.