Rip out setting of the log level from udev_new and put it in a new function

Rip out setting of the log level from udev_new and put it in a new function

This function is internal to elogind code, so external users of libudev
will not see those log messages. I think this is better. If we want to
allow that, the function could be put in libudev and exported.

v2: check that the string is more than one char before stripping quotes