Adds signals section to the manpage and generally improves contents and
wording

Allow slstatus to be used by programs that can grab status by calling
an external program on a periodic basis (e.g. tmux)

Even if we extend the flag to be an n-flag with a numerical input, we
already have other Unix-tools to take care of us much more flexibly,
e.g. with head(1) you can easily get the first 10 outputs with

	slstatus -s | head -n 10,

but also e.g. discard the first one and then get the consecutive 10
outputs.

For the X11-root-window-name, you can limit the runtime with timeout(1)
or a comparable solution.

This reverts commit fa8b939990ed0648f99b0cbeba4efed807160db3.

What we really want is to either output to WM_NAME or stdout. If we want
just one single line, we do slstatus | head -n 1.

d-flag: There's no need for that, use the &-operator or fork+exec in a
        non-shell-context. In the latter case you get the PID for free.
v-flag: If you want to find out which version of a package is installed,
        consult your package manager. That's his job.

We also change the semantics of -v to only return the version
information.
There is now no need for usage() to exit with anything other than 1.