1.4 master

Hi Graham,

When the users can select the directories to backup, it is very interesting to set a limit or a quota (to prevent abuse).
This patch add a quota feature to BURP.
There are two new options on the server side : soft_quota and hard_quota
If the estimated size (phase1_client) is greater than hard_quota, the server cancel the backup.
If the estimated size (phase1_client) is less than hard_quota, but greater than soft_quota, the server perform the backup, and a warning message is displayed.
These options can be overridden by the client configuration files  in  clientconfdir on the server. 
soft_quota=[b/Kb/Mb/Gb]
hard_quota=[b/Kb/Mb/Gb]
Example: 'hard_quota = 100Gb'. Set to 0 (the default) to have no limit.
This patch is a draft, i'm not sur that is the best implementation.
This is my first contribution to an open source project, I hope to use the right channel to propose this feature.

Best regards