isisd: refactor handling of SR Prefix-SIDs

isisd: refactor handling of SR Prefix-SIDs

Embed Prefix-SID information inside SPF data structures so that
Prefix-SIDs can be installed together with their associated routes
at the end of the SPF algorithm. This is different from the current
implementation where Prefix-SIDs are parsed and processed separately,
which is vastly suboptimal.

Advantages of the new code:
* No need to parse the LSPDB an additional time to detect and process
  SR-related changes;
* Routes are installed with their Prefix-SID labels in the same ZAPI
  message. This can prevent packet dropping for a few milliseconds
  after each SPF run if there are BGP-labeled routes (e.g. L3VPN) that
  recurse on IGP labeled routes;
* Much easier to support Anycast-SIDs, as the SPF code will naturally
  figure out the best nexthops and use only them (that can't be done
  in any reasonable way if the Prefix-SID Sub-TVLs are processed
  separately);
* Less code to maintain and reduced memory footprint;

The "show isis segment-routing prefix-sids" command was removed as
it doesn't make sense anymore now that "show isis route" exists.
Prefix-SIDs are a property of routes, so what was done was to extend
the "show isis route" command with a new "prefix-sid" option that
changes the output table to show the Prefix-SID information associated
to each route.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>