polypath: Plumb ifname_wanted `want' through privsep etc.

polypath: Plumb ifname_wanted `want' through privsep etc.

Prepare for there being multiple interface lists, and for a new kind
of `want'.  Specifically:

* Arrange to declare the type `struct interf_list'.
* Pass the want via the privsep protocol, as a character.
* Pass the want in lots of command line arguments.
* Move assert for rogue values to the use site, which is now in
  a different process so it can't be an assert.
* Introduce a variable `interfs' in polypath_record_ifaddr to allow
  it to manipulate a different list.
* Introduce a variable `max_interfs' in polypath_record_ifaddr to
  allow a different check.
* Print the relevant want in debugging output.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>