Fix Debian bug #478942: Fragile argument passing

Fix Debian bug #478942: Fragile argument passing

Fix some problems regarding the way check_disk_smb passes command line
arguments to smbclient(1).

| It runs:
|
| 	$res = qx/$smbclient "\/\/$host\/$share" $pass -W $workgroup \
| 	       -U $user $smbclientoptions -I $address -c ls/;
|
| [...]
|
| The documentation says that if the password is not passed, it
| defaults to "".  That is not true above, as $pass expands to
| nothing which leaves no argument at all (instead of an empty
| argument) so is different from providing with an empty password
| or with the -N option.
|
| Also, if the password starts with "-", you're in trouble, that's
| why -U $user%$pass may be prefered.
|
| Also, the doc says that if $user is not provided, then it
| defaults to "guest" but the problem is that if it is provided
| but empty, it is changed to "guest" as well, which prevents us
| from querying hosts that don't do user authentication.

[ http://bugs.debian.org/478942 ]

(Fixed by Stephane Chazelas, forwarded by Jan Wagner.)