dd-schroot-cmd: Relax session name check

dd-schroot-cmd: Relax session name check

As per schroot.conf(5):

  A  number  of characters or words are not permitted in a chroot name,
  session name or configuration filename.  The name may not contain a
  leading period (‘.’).  The characters ‘:’ (colon), ‘,’ (comma) and ‘/’
  (forward slash) are not permitted anywhere in the name.  The name may
  also not contain  a  trailing  tilde  (‘~’).

Relaxing the session name check in get_session_owner to better align
with schroot's actual restrictions reduces the chance that a user will
create a session yet be unable to manipulate it with dd-schroot-cmd.

Signed-off-by: James McCoy <jamessan@debian.org>
Signed-off-by: Paul Wise <pabs@debian.org>