top(1): modernize a bit; reduce warnings

top(1): modernize a bit; reduce warnings

- Replace caddr_t with "void *". This reduces
the number of warnings at WARNS=6
- use "static" where possible
- sprinkle const where possible

This leaves at WARNS=6:
35 warnings in top.c
72 warnings in machine.c
5 warnings in commands.c

all of which are either "incompatible-pointer-types-discards-qualifiers"
or "cast-qual"