MFC r306530, r306531, r306532, r306579:

r306530:
Declare a module for evdev and add dependency to ukbd(4) and ums(4)

Prepare for making evdev a module. "Pure" evdev device drivers (like
touchscreen) and evdev itself can be built as a modules regardless of
"options EVDEV" in kernel config. So if people does not require evdev
functionality in hybrid drivers like ums and ukbd they can, for instance,
kldload evdev and utouchscreen to run FreeBSD in kiosk mode.

r306531:
Add dependency to evdev module (if required)

r306532:
Add dependency to evdev module

r306579:
Modularize evdev

• Convert "options EVDEV" to "device evdev" and "device uinput", add modules for both new devices. They are isolated subsystems and do not require any compile-time changes to general kernel subsytems
• For hybrid drivers that have evdev as an optional way to deliver input events add option EVDEV_SUPPORT. Update all existing hybrid drivers to use it instead of EVDEV
• Remove no-op DECLARE_MODULE in evdev, it's not required, MODULE_VERSION is enough
• Add evdev module dependency to uinput

Submitted by: Vladimir Kondratiev <wulf@cicgroup.ru>