ixl(4): Add ability to control link state on ifconfig down

Add sysctl link_active_on_if_down, which allows user to control
if interface is kept in active state when it is brought
down with ifconfig. Set it to enabled by default to preserve
backwards compatibility.

Reviewed by: erj
Tested by: gowtham.kumar.ks@intel.com
Sponsored by: Intel Corporation
Differential Revision: https://reviews.freebsd.org/D28028