Disallow use of undocumented snapshot delimiters

Disallow use of undocumented snapshot delimiters

Since the introduction of selective snapshot format configuration in
dc3c9b5 (allow selective enabling of snapshots, 2007-07-21), we allowed
seven different delimiters for snapshot formats, while the documentation
has always been clear about spaces being the only valid delimiter:

    The value is a space-separated list of zero or more of the values
    "tar", "tar.gz", "tar.bz2", "tar.xz" and "zip".

Supporting the undocumented delimiters makes the code unnecessarily
complex. Remove them.

Signed-off-by: Lukas Fleischer <cgit@cryptocrack.de>