plugins/sixaxis: Add a set_leds_sysfs() function

plugins/sixaxis: Add a set_leds_sysfs() function

On recent kernels the hid-sony driver exposes leds class entries in
sysfs for setting the Sixaxis LEDs, use this interface and fall back to
hidraw in case using sysfs fails (e.g. on older hid-sony versions).

Setting the LEDs via sysfs is the preferred way on newer kernels, the
rationale behind that is:

  1. the Sixaxis uses the same HID output report for setting both LEDs
     and rumble effects;
  2. hid-sony remembers the state of LEDs in order to preserve them when
     setting rumble effects;
  3. when the LEDs are set via hidraw hid-sony has no way to know the
     new LEDs state and thus can change the LEDs in an inconsistent way
     when setting rumble effects later.

Also require libudev >= 172, this is where
udev_enumerate_add_match_parent() has been first introduced.

NOTE: using udev_enumerate_add_match_parent() results in a memory leak
when enumerating child devices, this has been fixed in udev 207; the
commit which fixes the issue is this one:
http://cgit.freedesktop.org/systemd/systemd/commit/?id=51cc07576e119dea6e65478eeba9472979fd0936