There's a difference between

    cmd | wc -l

and

    x="$(cmd)"; echo "$x" | wc -l

in that the assignment in the latter removes a trailing newline **if one
exists**, but the "echo" always adds one. This means that if `cmd`
produces no output, the former has 0 lines, and the latter has 1 (empty)
line.

I've not found an easy way to fix this in a way that `wc` can tell the
difference between an empty line and a non-empty one, so I had to find a
separate way to count non-empty lines.

I did try `echo "$x" | grep . | wc -l`, but `shellcheck` complains with
[SC2126](https://github.com/koalaman/shellcheck/wiki/SC2126). The
suggested alternative, `echo "$x" | grep -c .`, causes a failure on
empty inputs (i.e. when there are no upgrades) because `grep` exits with
a failure status if no lines are selected.

My options were then to go with `grep | wc` and silence the warning, or
do `grep -c | cat` to mask the `grep` failure (again). Given that they
both have the same number of commands in the pipe, but `cat` is probably
*slightly* cheaper than `wc`, and the `cat` version doesn't require
adding a `shellcheck`-silencing comment, let's go with that.

You have to explicitly enable it though

The notification is supposed to read "Reboot when it is convenient - to
install the updates", but I kept reading it as "Reboot - when it is
convenient to install the updates.", and I couldn't figure out the
"correct" punctuation to do that while remaining proper English. So move
the wording about instead.

Pipe success is determined by the success of the last command in the
pipe, and `grep` indicates failure if no lines were selected. If we
don't download anything then `grep` fails, the pipe fails, and the
script - due to `set -e` - then fails/aborts at that point.

So pipe through `cat` last, which always succeeds.

Don't care about the "Reading package lists..." and other interactive
fripperies, or the list of packages no longer required, or the full list
of packages to be upgraded, every time we just want to download new
updates. But logging the actual downloads is kinda useful, so keep
those.

To allow for kernel upgrades, which install new packages.

Also, make the way we pass options to `apt-get upgrade` more consistent
between calls. Notably, put all options before the command, as indicated
by the apt-get(1) man page.