compositor: support loading backend via shortened name

While the --backend parameter looks like it takes a file name, it really
is selected from a list of supported strings that are then funneled
through a translation to enum weston_compositor_backend [1].

Because all backend parameters are of the form "...-backend.so", and
writing "--backend=...-backend.so" is boring, allow the --backend option
to match the backend name without "-backend.so" suffix instead.

For example, this allows to use "--backend=headless" instead of
"--backend=headless-backend.so".

Update help text and documentation. Keep the old way working for
backwards compatibility.

[1] 50dbf385 ("libweston: use enum to choose the backend")

Signed-off-by: Philipp Zabel <philipp.zabel@gmail.com>