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>