o Explicitly set the close-on-exec flag of descriptor 3 before
exec()ing. Tidy up file dups in general prior to exec(). This prevents our tun device (fd 3) from staying open (and configured) despite handing off all it's links and exiting (because ``cat'' holds it open).
o Don't bother SIG_DFL'ing signals before exec() as they're
already trapped with specific handlers and will be handled correctly by the exec.
o Use values from paths.h for "/dev/" and "/dev/tty".
o Don't assert() in physical.c.