src.ctf.fs: name the stream objects with their file's path

src.ctf.fs: name the stream objects with their file's path

This patch makes a src.ctf.fs component name its stream objects. The
name of a stream object is the full path of the first file of its
backing data stream file group.

The purpose of those names is for sink.text.pretty to indicate in which
file (or which file group) events or packets were discarded by the
tracer.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>