Patch for bugzilla #12050

Please find appended the solution.

Short description of the problem:

After adding a new lease, you are in editing mode. All fields are set right, but a second click ('update' ) is necessary. With determining the exact mode 'add/edit' this second click can be avoided for new entries ( new MAC/IP combination).

The patch adds determination of add/edit mode. If the MAC/IP pair exists in the list of fixed leases edit mode is selected, add mode is selected else.

Sorry, did not produce the patch from git but by diffing changed file and the core 129 original.

-Bernhard