Silence top(1) compiler warnings

The contrib/top code is no longer maintained upstream (last pulled 16 years
ago). The K&R-style followed by the code spews -Wimplicit-int and -Wreturn-type
warnings, amongst others. This silences 131 warnings with as little modification
as possible by adding necessary return types, definitions, headers, and header
guards, and missing header includes.

The 5 warnings that remain are due to undeclared ncurses references. I didn't
include curses.h and term.h because there are several local functions and macros
that conflict with those definitions.

MFC after: 3 weeks
Reviewed by: cem, ngie
Submitted by: Randy Westlund <rwestlun@gmail.com>
Differential Revision: https://reviews.freebsd.org/D6468