Start to implement ifnet DDB support:

• 'show ifnets' prints a list of ifnet *s per virtual network stack,
• 'show ifnet <struct ifnet *>' prints fields matching the given ifp.

We do not yet print the complete set of fields and might want to
factor this out to an extra if_debug.c file in case this grows
a lot[1]. We may also want to grow 'show ifnet <if_xname>' support[1].

Sponsored by: ISPsystem
Suggested by: rwatson [1]
Reviewed by: rwatson
MFC after: 5 days