sheepdog: Fix snapshot ID parsing in _open(), _create, _goto()

sd_parse_uri() and sd_snapshot_goto() screw up error checking after
strtoul(), and truncate long tag names silently.  Fix by replacing
those parts by new sd_parse_snapid_or_tag(), which checks more
carefully.

sd_snapshot_delete() also parses snapshot IDs, but is currently too
broken for me to touch.  Mark TODO.

Two calls of strtol() without error checking remain in
parse_redundancy().  Mark them FIXME.

More silent truncation of configuration strings remains elsewhere.
Not marked.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
