This is simpler and avoids issues with changing router-ids.

This increases the chances for a stable router-id in case the set of
interfaces changes.

This fixes compilation under musl.

This fixes compilation under musl.

Based on the kernel version, can be overridden explicitly.

Prefixes are counted in bits, not in bytes.  It is safe to compare all
16 bytes, since prefixes are always normalised -- they have zeroes beyond
the prefix length, see mask_prefix.  Alternatively, we could use
prefix_cmp.

Was used in the first version of the disambiguation.c algorithm.

This reverts commit 9edf0a14.