dhcpcd-11 will be privsep only

[rsmarples]

The privsep implementation dhcpcd offers has proven robust and has been the default since dhcpcd-10.
To simplify the code path, the option to NOT have privsep will be removed to simplify the code path even more.
For Linux, we still have the --disable-seccomp configure option when something in libc changes we haven't codified yet.

This idea is currently blocked by #278 as I do like having IllumOS as a supported OS.

[jxcang]

Please see #87 (comment)

If we could agree on how to improve privsep to be more utilizing libcap and systemd compatible, I would appreciate that.
Right now, as I have discussed in the above comment, would over-privileging some proxy processes, or over-privileging the privileged process.