mpr: Handle errors from copyout() in ioctl handlers

In preparation for adding a __result_use_check annotation to copyin()
and related functions, start checking for errors from copyout() in
the mpr(4) user command handler. This should make it easier to catch
bugs.

Reviewed by: imp, asomers
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D43177