Rework compat shims in ifioctl().

Centralize logic for handling compat ioctls into two blocks of code at
the start and end of the ioctl routine. This avoids the conversion
logic being spread out both in multiple blocks in ifioctl as well as
various helper functions.

Reviewed by: brooks, kib
Obtained from: CheriBSD
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D29891