segtree: add expr_to_intervals()

segtree: add expr_to_intervals()

Refactor code to add the new expr_to_intervals(). This function takes
the list of set element expressions and convert them to a list of
half-closed intervals.

This is useful for different purposes, such as interval overlap
and conflicts detection.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>