fs-sink: add the single-trace option

This option allows the user to control the output directory of the trace
written (instead of the path being derived from the source trace path
and hostname). This implies that the component only receives one trace.

If the output directory exists and is not empty, an error is returned.

To use it:
--component sink.ctf.fs --path /tmp/out -p single-trace=true

Signed-off-by: Julien Desfossez <jdesfossez@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>