Make it possible to disable NOP-In PDUs by the iSCSI initiator by setting
kern.cam.ctl.iscsi.ping_timeout to 0. This fixes interoperability with
some initiators that don't properly support NOP-Ins, namely iPXE/gPXE.

Submitted by: Chen Wen <pokkys@gmail.com>
MFC after: 1 week
Sponsored by: The FreeBSD Foundation