Do not allow use create keyword as hostname when ifconfig(8) is invoked
for already existing interface.

It appeared, that ifconfig(8) assumes create keyword as hostname and
tries to resolve it, when ifconfig ifname create invoked for already
existing interface. This can produce some unexpected results, when hostname
resolving has successfully happened. This patch adds check for such case.
When an interface is already exists, and create is only one argument,
return error message. But when there are some other arguments, just remove
create keyword from the arguments list.

Obtained from: Yandex LLC
MFC after: 3 weeks
Sponsored by: Yandex LLC
Differential Revision: https://reviews.freebsd.org/D17171