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

[ae]

Summary

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 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.

[yuripv]

head/sbin/ifconfig/ifconfig.c

515

grammar nit: exists