ifconfig: abort if loading a module fails other than for ENOENT

[asomers]

Summary

If "ifconfig create" tries to load a kernel module, and the module
exists but can't be loaded, fail the command with a useful error
message. This is helpful, for example, when trying to create a cloned
interface in a vnet jail. But ignore ENOENT, because sometimes ifconfig
can't correctly guess the name of the required kernel module.

MFC after: 2 weeks

Test Plan

Manually tested

[jhb]

Is this related to the issues you had with using if_wg(4) interfaces in jails?

[asomers]

In D37873#861872, @jhb wrote:

Is this related to the issues you had with using if_wg(4) interfaces in jails?

Precisely.