exit_nicely() exits from the guile thread before it gets a chance
to exit() from the process as a whole.

And there's really nothing for the reporting-code in exit_nicely
to actually report at this point, anyway.

This handler got broken when exit_nicely stopped actually triggering
an exit from the entire process and started just exiting the guile thread.

And, actually, it never exited with an error-status before--
which it does now.

Streams that start out connected to non-tty FDs default to fully-buffered
and retain that configuration after those underlying FDs are redirected,
which prevents the "need to run from an xterm" fix from actually working
unless we do something like this to reset the stream buffers.

For those of us with difficulty seeing red,
it's just too hard to tell which way the robot is facing
when the only distinction is whether which of the side bumps
is red vs. grey (especially when it's just a handful of pixels).

Yet another of the niceties that comes out of just using guile's REPL
is that readline support is just figured out when building guile;
so by the time gnurobots is being built, readline support is either
there or it isn't--and even if our guile isn't readline-enabled,
we can still just run without readline (though, in practice,
I don't think I've ever seen a build of guile in the last 15 years
that didn't have readline...).

Since this GLib 1.35.1 was released >4 years ago
(which is presumably old enough to depend on it),
and the alternative is to drop Bradley's -Werror from AM_CFLAGS
and go back to using deprecated APIs.

Looks like these haven't been necessary since probably pkg-config 0.16,
12 years ago.