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

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
Reviewed by: jhb
Differential Revision: https://reviews.freebsd.org/D37873