Enhance rc.d/netwait script to wait for late-attaching interfaces such as USB NICs.

[Ian Lepore]

Based on patches submitted by  Brenden Molloy I've put together a
phabricator review for enhancements to the /etc/rc.d/netwait script so
that it can be used to wait for one or more USB network interfaces to
appear before continuing with post-NETWORKING rc scripts.  Anyone
interested in testing or reviewing the changes, please see

https://reviews.freebsd.org/D4608

-- Ian