iflib: Add support for SIOCGIFDOWNREASON ioctl

This change adds native support for the SIOCGIFDOWNREASON ioctl in iflib.

When ifconfig issues SIOCGIFDOWNREASON, the request is now routed through a
new driver callback (IFDI_GET_DOWNREASON). iflib allocates the ifdownreason
structure, calls the driver to fill the down-reason message, and then
returns the data back to ifconfig for display.

Without this change, iflib-based drivers cannot implement link-down reason
reporting even if the hardware provides the information.

No functional change for existing drivers unless they implement the new
IFDI_GET_DOWNREASON method. Existing drivers continue to behave as before.

Reviewed by: gallatin, erj, kgalazka, ssaxena, iflib
Differential Revision: https://reviews.freebsd.org/D54045
MFC After: 1 week