Add crm_claim_merge, allows to merge claims.

I read again the code, and that's not the same thing, _merge_get_default_main is to get the claim we'll merge the others in, and _merge_sort gives the priority for the merges of the other claims, so _merge_sort receives a list where the the main claim has already been removed. I find it confusing to call _merge_sort from _merge_get_default_main because it has not the same purpose, even if by chance they have the same sorting.