replace SRP with SMR in the if_idxmap.

replace SRP with SMR in the if_idxmap.

when i first wrote if_idxmap i didn't realise (and no one thought
to tell me) that index 0 was special and means "no interface", so
while here use the 0th slot in the interface map to store the length
of the map instead of prepending the map with a length field.
if_get() now special cases index 0 and returns NULL directly. this
also means the size of the map is now always a power of 2, which
is a nicer fit with what the kernel malloc aprovides.

tweaks and ok visa@