rename radvdion.c readn_writen.c
syscalls_test.c: flesh out radvd_bind

just return 0 for success
add config.h.in~ to .gitignore
always write a pid file if specified on command line

When running tests, we need a reliable way to find the radvd pid so we can
reliably kill radvd.  Write a pidfile, or die, if the -p option is passed
to radvd.
add trivial radvd run check