Fix useless fields in interface.h.
Pass NULL instead of zeroes.

We pass NULL here because prefix is NULL.
Fix assertion.
Parse Source-Specific Update TLV.
Compare source-specific buffered updates.
Format Source-Specific Update TLV.
Allow flushing source-specific updates.
Allow specific updates to be bufferised.
Add some guarded code to handle requests in a near future.
Improve route printing with "from" field.
Add local interface source-specific printing.

It also update the route's id.
Add options to control the table and priority numbers.
Add filter to match source addresses.

Add 3 new options to configuration and manpages:
src-ip, src-eq, src-le and src-ge.
Set the source prefixes through filters.

Since it's a route attribute, it fits better there.
Format TLV for (standard) requests.
Format TLV for unicast requests.
Format TLV for multihop requests.
Format TLV for unicast multihop requests.
Make send_request_resend deal with source prefixes.
Make handle_request deal with source prefixes.