bthidd(8): Add evdev protocol support for bluetooth keyboards and mouses

bthidd(8): Add evdev protocol support for bluetooth keyboards and mouses

User-visible changes:

"-u" is added to to list of command line options supported by bthidd.
Use it to enable evdev support. uinput and evdev modules should be
kld-loaded or compiled into the kernel in that case.

bthidd_evdev_support rc.conf variable is added to control enabling of
evdev support in bthidd startup script. Possible values are: "YES", "NO",
"AUTO"(default). Setting bthidd_evdev_support to "AUTO" inserts "-u" option
if kernel is compiled with EVDEV_SUPPORT option enabled.

Support for consumer HID usage page keyboard events is implemented. Most of
them are available only through evdev protocol.

kern.evdev.rcpt_mask sysctl is checked, so "sysctl kern.evdev.rcpt_mask=12"
should be executed if EVDEV_SUPPORT is compiled into kernel.

It is recommended to regenerate bthidd.conf entries with bthidcontrol(8)
"Query" command to set user-friendly names of bluetooth devices.

Reviewed by:	emax, gonzo, wblock (docs), bcr (docs, early version)
Differential Revision:	https://reviews.freebsd.org/D13456