non-zero process exit status when too many packets are unreturned

The number of unreturned packets in excess of 50% is summed over all
hosts.  If this is nonzero, this count plus 1 is the process exit
status.
oping: Implement the "-Z" option.

This makes it possible to configure the threshold for exiting with a non-zero
exit status due to missing replies.
fiddle with failure threshold messages and comments
oping: Make the exit status the number of hosts failed.

As far as I know, EXIT_FAILURE is only portable when used as-is, i.e. without
adding anything to it. Especially with "EXIT_FAILURE == -1" we might run into
trouble. I think it's more convenient to just use the number of failed hosts
rather than 1+<num> (on Linux).
implement some prettyping-inspired display instead of textual

this is a first stab at porting "prettyping.sh" into noping. the idea
is to show a histogram of ping times instead of numerical values. with
some work, we could actually show both, but this focuses on
implementing the hard part (the histogram) properly.

it is very rudimentary for now:

 * the math may be wrong for the size of the bar, I was mostly in a
   rush to make unicode work and have something pretty quickly.

 * all hosts are on the same line: each should have its own line

 * the histogram should be displayed in a separate window

 * it should use background colors as well

 * it should fallback when the terminal is not unicode-capable

 * the scaling logic is fully automatic, which necessarily gives weird
   results at first

this requires switching to the ncursesw library and other unicode
ncurses hackery described in this post:

http://newsgroups.derkeiler.com/Archive/Rec/rec.games.roguelike.development/2010-09/msg00050.html

pretty ping is a awk/bash script, MIT-licensed, available here:

https://bitbucket.org/denilsonsa/small_scripts/src/tip/prettyping.sh
put histograms into the independent window, to eventually restore ping times display
restore usual ping display, now that histograms are in a different window
make histogram colors with a nicer background
remove a little quirk i introduced by mistake

this was breaking the border
wrap around the window once the histogram grows too big
cleanup safety checks

make sure that if we go beyond the max ratio, we just mark it as one
rework scaling algorithm so it covers the whole range

i did this after some stress testing, now it covers the whole range on input
better wrapping: add a cursor and respect inner borders
switch to ncursesw in the build chain
Refactor the graph printing into its own function.

This makes it much easier and cleaner to add a second, non-Unicode
implementation and possibly even a non-color version.
autodetect unicode, and fallback to ACS scancodes on failure

this is fairly ugly - we use a hidden ncurses function to detect
unicode (but at least we don't reimplement it), then we mess around
with the character arrays to display the rights symbols

i am not sure i like this, but i prefer to braindump than to stall, so
let's move forward.
move unicode detection to a separate function

this will ease transition to a commandline option override
add commandline flag for forcing or disabling unicode

note that "forcing" may not work: ncurses still expects your locale to
be correct, so maybe the force flag should also explicitely set a utf8
locale?
src/oping.h: Don't use the ncurses internal _nc_unicode_locale() function.

The "-u" and "-U" options are now handled in a single variable so that the
last option "wins".
Add documentation for the "-u" and "-U" options.