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
88 warnings in machine.c
7 warnings in commands.c

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