Add support for SCM_MONOTONIC as a control message for FreeBSD.

[recatek]

On FreeBSD using SO_TS_MONOTONIC (see #2058) on a socket allows you receive timestamps as a monotonic timespec. The support for the SO_TS_CLOCK switch was added to nix some time ago, but not the way to access the data as a control message. Adding ScmMonotonic to nix::sys::socket::ControlMessageOwned would close the loop here.