This fixes a format string vulnerabilty.

evdev_log_message() composes a format string consisting of a fixed
prefix (including the rendered device name) and the passed-in format
buffer. This format string is then passed with the arguments to the
actual log handler, which usually and eventually ends up being printf.

If the device name contains a printf-style format directive, these ended
up in the format string and thus get interpreted correctly, e.g. for a
device "Foo%sBar" the log message vs printf invocation ends up being:
  evdev_log_message(device, "some message %s", "some argument");
  printf("event9 - Foo%sBar: some message %s", "some argument");

This can enable an attacker to execute malicious code with the
privileges of the process using libinput.

To exploit this, an attacker needs to be able to create a kernel device
with a malicious name, e.g. through /dev/uinput or a Bluetooth device.

To fix this, convert any potential format directives in the device name
by duplicating percentages.

Pre-rendering the device to avoid the issue altogether would be nicer
but the current log level hooks do not easily allow for this. The device
name is the only user-controlled part of the format string.

A second potential issue is the sysname of the device which is also
sanitized.

This issue was found by Albin Eldstål-Ahrens and Benjamin Svensson from
Assured AB, and independently by Lukas Lamster.

Fixes #752



Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>