monitor: Fix output for ranges in anonymous sets

monitor: Fix output for ranges in anonymous sets

Previous fix for named interval sets was simply wrong: Instead of
limiting decomposing to anonymous interval sets, it effectively disabled
it entirely.

Since code needs to check for both interval and anonymous bits
separately, introduce set_is_interval() helper to keep the code
readable.

Also extend test case to assert ranges in anonymous sets are correctly
printed by echo or monitor modes. Without this fix, range boundaries are
printed as individual set elements.

Fixes: 5d57fa3e99bb9 ("monitor: Do not decompose non-anonymous sets")
Signed-off-by: Phil Sutter <phil@nwl.cc>
Reviewed-by: Pablo Neira Ayuso <pablo@netfilter.org>