MFC r353128-r353129: fully initialize cloned devices w/ make_dev_args

r353128:
kern_conf: fully initialize cloned devices with make_dev_args, too

Attempting to initialize si_drv{1,2} with mda_si_drv{1,2} does not work if
you are operating on cloned devices.

clone_create must be called prior to the make_dev* family to create/return
the device on the clonelist as needed. This device is later returned early
in newdev(), prior to si_drv{0,1,2} initialization.

This patch simply breaks out of the loop if we've found a device and
finishes init.

r353129:
Remove the remnants of SI_CHEAPCLONE

SI_CHEAPCLONE was introduced in r66067 for use with cloned bpfs. It was
later also used in tty, tun, tap at points. The rough timeline for being
removed in each of these is as follows:

• r181690: bpf switched to use cdevpriv API by ed@
• r181905: ed@ rewrote the TTY later to be mpsafe
• r204464: kib@ removes it from tun/tap, declaring it unused

I've not yet been able to dig up any other consumers in the intervening 9
years. It is no longer set on any devices in the tree and leaves an
interesting situation in make_dev_sv where we're ok with the device already
being set SI_NAMED.