CIFS: Fix cred option not detected + remove checking for cred options co...

...mpletely so that users are not stuck with the guest mount option when they want to use other then the cred cifs mount option. Only use guest option by default when no other BACKUP_OPTIONS are given.

Dear, for security reasons I did not want to store my credentials in plaintext in the backup. So I wanted to give the cifs options user so that mount will interactively asks my password. This was not possible with the current code. Additionally I found that when even giving the cred option to cifs, this was not detected by the current code. In other words it will still use the fallback guest option, even when cred option is set by the user in BACKUP_OPTIONS. Thanks