top: disable tty scrollback buffer to improve SIGWINCH

A scrollback buffer used in several terminal emulators
could be a real inconvenience to a user following some
resize operations. Extra keystroke(s) would frequently
be required in order to properly render top's display.

After much sleuthing we unearthed two terminfo strings
which have the effect of disabling/restoring that darn
scrollback buffer. They were well hidden under a title
of strings 'to start/end programs using cup'. In turn,
'cup' deals with a tty's cursor addressing capability.

We don't care what you call them or what they refer to
so long as they get the job done. And these really do!
Be advised, however, that there are some side effects,
several of which can even be considered as beneficial:

. enter_ca_mode/smcup/ti disables scrollback buffering
( and that's good, it's what we had always hoped for )

. exit_ca_mode/rmcup/te restores the scrollback buffer
( but also restores screen contents existing pre-top )
( which is different from former program end results )
( where that last rendered screen was left untouched )

. the above screen replacement would impact ^Z suspend
( thus we keep the scrollback buffer disabled during )
( the suspend/resume sequence so that the users will )
( have a visual clue that top is suspended not ended )

If a terminal does not support these terminfo strings,
we will revert to top's former behavior at program end
where we position the cursor at screen bottom and then
output a single newline character. This will prevent a
shell prompt from embedding within top's final screen.

This commit's approach has been tested under a variety
of emulators and window managers, many of which linked
with libvte and others that employed their own scheme.
Examples are: gnome_terminal; kde konsole; lxterminal;
terminator; terminology; urxvt; xfce4-terminal; xterm.

I do now believe that the whole SIGWINCH deal is done!

(everything is perfectly justified plus right margins)
(are completely filled, but of course it must be luck)

Reference(s):
http://www.freelists.org/post/procps/top-won-the-sigwinch-war
http://www.freelists.org/post/procps/top-won-the-sigwinch-war,4



Signed-off-by: Jim Warner <james.warner@comcast.net>