syscalls_test.c: write to the test socket

Try to start communicating to radvd from the testing layer
radvd.c: validate return values from recv_rs_ra

The testing layer has exposed some places where more error handling can be
used.
recv.c: Removing vestigial select

This code should not be here.  Radvd will only call into this function
when poll says there is something to read.