psm: add support for evdev protocol

Both relative and absolute multitouch modes are supported.
To enable psm(4) evdev support one should:

1. Add device evdev and options EVDEV_SUPPORT to kernel config file
2. Add hw.psm.elantech_support=1 or hw.psm.synaptics_support=1 to /boot/loader.conf for activation of absolute mode on touchpads
3. Add kern.evdev.rcpt_mask=12 to /etc/sysctl.conf to enable psm event sourcing and disable sysmouse

Reviewed by: gonzo
Approved by: gonzo (mentor)
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D10265
Tested by: wulf, Jan Kokemueller (Lenovo devs)