Make debug logging dynamically controllable with $UMOCKDEV_DEBUG

This avoids having to rebuild umockdev to debug.

Add documentation to README.rst
Emulate isatty()

It would previously return true for emulated devices as PTYs are being used
internally, but the actual emulated device is not always supposed to be a TTY.
Fixes regression with X.org synaptics > 1.8 device emulation as that uses
xf86OpenSerial() to open evdev devices (which is wrong, but reality).
Refresh synaptics-touchpad record
Don't fall back to real ioctls on emulated fds

Some ioctls (e. g. for ttys) work on the emulated device nodes which shouldn't
for real ones.