RFC3095 list compression: factorize removal mask computation
RFC3095 list compression: factorize removal mask computation

The list compression types 2 and 3 use a removal mask. This change
factorizes that code in a new dedicated function.

didier-barvaux/rohc,didier-barvaux/rohc,didier-barvaux/rohc,didier-barvaux/rohc,didier-barvaux/rohc