Fix: channel names are not validated

This patch ensures:

  1. A channel name does not contain any '/' character, since
     relative paths may be injected in the channel name
     otherwise (knowing that the channel name is eventually
     part of a file name)
  2. A channel name does not start with a '.' character, since
     trace readers (Babeltrace is one of them) could interpret
     files starting with a dot as hidden files and ignore
     them when opening the CTF trace

Fixes: #751
Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>