This leads to the revert of r355806; this reduces duplication in keyboard
registration and driver switch lookup and leaves us with one authoritative
source for currently registered drivers. The reduced duplication later is
nice as we have more procedure involved in keyboard setup.
keyboard_driver->flags is used to more quickly detect bogus adds/removes.
From KPI consumers' perspective, nothing changes- kbd_add_driver of an
already-registered driver will succeed, and a single kbd_delete_driver will
later remove it as expected. In contrast to historical behavior,
kbd_delete_driver on a driver registered via linker set will now actually
de-register the driver so that it may not be used -- e.g. if kbdmux's
MOD_LOAD handler fails somewhere.
Detection for already-registered drivers in kbd_add_driver has improved, as
the previous SLIST_NEXT(driver) != NULL check would not have caught a driver
that's at the tail end.
kbdinit is now called from cninit() rather than via SYSINIT so that keyboard
drivers are available as early as console drivers. This is particularly
important as cnprobe will, in both syscons and vt, attempt to do any early
configuration of keyboard drivers built-in (see: kbd_configure).
Reviewed by: imp (earlier version, pre-cninit change)
Differential Revision: https://reviews.freebsd.org/D22835
3ed7166aca