evdev: several minor fixes and improvements

[wulf]

Summary

1.Hide "kern.evdev.rcpt_mask" sysctl if kernel is compiled w/o EVDEV_SUPPORT

It can be used for discovery if evdev support for hybrid devices is compiled
into kernel or not. Hide "kern.evdev.sysmouse_t_axis" sysctl as well.

2. Lock Giant around keyboard ioctls This fixes turning ukbd(4) LEDs on/off with evdev interface as well
3. Take driver's lock in cdev write handler if necessary
4. Fixes to ums-style sysmouse(4) T-axis support Do not report T-axis wheel as button presses Reverse T-axis to match Linux Remove wrong comment
5. Disable value normalization and state filtering for SND events. Some events can take sound pitch as a value so can not be represented as binary on/off events. Tracking for on/off state is left in place as it is a part of the evdev API.
6. Do not start/stop softrepeat callout if no clients attached.

[wulf]

Unfortunately, phabricator hangs if I try to upload full-context diff, so standard 3-line context diff is uploaded.

[gonzo]

LGTM