Initial commit on integrated BIRD

New data types net_addr and variants (in lib/net.h) describing
network addresses (prefix/pxlen). Modifications of FIB structures
to handle these data types and changing everything to use these
data types instead of prefix/pxlen pairs where possible.

The commit is WiP, some protocols are not yet updated (BGP, Kernel),
and the code contains some temporary scaffolding.

Comments are welcome.