Change network configuration to use macros.

Change network configuration to use macros.

Change the network configuration so everything is set using
some macros at the top of src/guestfs.c.

Also, rename the macros used in the daemon so they are not the
same.  It was a very long time since these sets of macros had to
match the ones defined in src/guestfs.c, despite what the comment
said.

Note that this commit should not change the semantics of the
program at all.