Fix definition of available path

[huitema]

The check for available paths was too simplistic, and only tested for availability of CID for path_id=0. This caused the code to return an error 1099 if the application tried to open a new path after path 0 was abandoned.

Added two test cases in the multipath test suite:

• test multipath_break3 simulates a disconnected interface, and thus an address unreachable signal for the path. Verify that path 0 is abandoned after that.
• test multipath_back0 also simulates the disconnection of the link under path0, but also simulate that interface going back up and the application requesting the creation of a new path.

The fix required checking the test multipath_ab1, which simulates abandoning a series of path_id before the corresponding path was activated. We verify that the test for path number availability checks for this kind of deletion and skips path ids for which the CID were defined and then deleted.

[huitema]

Close issue #1918

[akh7177]

This indeed fixes the issue ✨Thanks a lot @huitema