Currently, less(1) uses K&R prototypes, which both fails to provide useful
compiler-time type checking, and also causes problems for targets where
multiple incompatible calling conventions may be selected based on argument
types. This change switches less(1) to ANSI prototypes.

While there, we also remove use of "register", and attempt to use "const" a
bit better now that the compiler can check argument types.

Reviewed by: cem, emaste
MFC after: 3 weeks
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D10152