babeld: Take interface name from "option ifname"

babeld: Take interface name from "option ifname"

Previously, we were using the name of the section as the interface name,
but this has several drawbacks:

- no "-" or other special characters are allowed in a section name,
  preventing some physical interface names to be specified this way
  (e.g. "tun-topeka" would not work)
- it is harder to support default interface options (no interface name)

This change is not backward compatible, but a later commit will fix that.