Don't allow slashes and dots in overriden trace chunk names

Don't allow slashes and dots in overriden trace chunk names

A trace chunk's name is used when building its path. Therefore,
we ensure that a chunk's name can never contain '/' or '.' in
order to prevent malicious trace chunk names that would escape
a session's output directory.

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