carp: allow commands to use interface name rather than index

Get/set commands can now choose to provide the interface name rather
than the interface index. This allows userspace to avoid a call to
if_nametoindex().

Suggested by: melifaro
Reviewed by: melifaro
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D39359