This reverts commit bf8555b2.

Makes it consistent with the other interface flags.

Note that this commit is a no-op, since this condition is true by
default, and it cannot be set in the configuration yet.

Note that this commit effectively disables the computation of a cost
from the RTT, since 'max-rtt-penalty' defaults to 0.

This avoids a crash when an interface is added both in the configuration
file and on the command-line.

Interface_up can be called before an IPv6 address has been assigned to
the interface.  In this case, we need to down the interface again, wait
for a few seconds, then attempt to up it again.

They are no longer used except for configuring interfaces with no
explicit configuration.

This feature was little used, and complicated the code somewhat.

This avoids warnings about being unable to determine things about
nonexistent interfaces.

The old name conflicts with autoconf-generated files.

Noted by Matthieu Boutier.

Noted by Matthieu Boutier.