top(1): Support wider terminals (Bug 2448)

Bump MAX_COLS from value of 255 to 512 to support wider terminals.
A similar change was also done on FreeBSD 5 months ago (r244180)

Reported-by: Charles Rapenne
https://bugs.dragonflybsd.org/issues/2448
top(1): Remove whitespace from local modifications

This only cleans up code we added, there are no functional changes.