rc: Improve load_kld

[des]

Summary

• Centralize the usage message.

• Document and enforce that -e and -m are mutually exclusive; previously, speficying both would result in only -e being applied.

• If -e was not specified, and -m was not specified or did not match, fall back to kldstat -n file which will always work for modules that aren't built into the kernel.

This means the kld and ntpd scripts can now rely on load_kld to dtrt.

MFC after: 1 week