Pass all parameters to -net tap,... as env vars to if script

Pass all parameters to -net tap,... as env vars to if script

qemu_ifup and qemu_ifdown now get in environment variables the values
of all of the settings specified on the command line.  Each parameter
<foo> actually specified appears in an environment variable
QEMU_IF_<foo>.  (NB this variable typically has some lowercase
letters.)

Currently only parameters actually specified appear in the
environment; settings which take their default values are not provided
to the scripts.