Fixed parsing of mount options when doing DFS submount

Since these hit the same routines, and are relatively small, it is easier to review
them as one patch.

Fixed incorrect handling of the last option in some cases
Fixed prefixpath handling convert path_consumed into host depended string length (in bytes)
Use non default separator if it is provided in the original mount options

Acked-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Igor Mammedov <niallain@gmail.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>