Provide proper contemporary function prototypes for many of the functions

Provide proper contemporary function prototypes for many of the functions
implemented in top(1), rather than relying on K&R prototypes, which can
cause problems on targets where there are multiple incompatible calling
conventions and the compiler requires argument information to select the
correct one.

(There's a bit more to do here, since it looks like top(1) also sometimes
provides prototypes for various curses functions rather than relying on
the header file...)

Sponsored by:	DARPA, AFRL
MFC after:	1 week