udev: Introduce UDEV_PROPAGATE_LOG macro (#5302)

udev: Introduce UDEV_PROPAGATE_LOG macro (#5302)

As per commit 25e773e "udev: switch to systemd logging functions"
Now log_set_max_level() in udev_new() overwites system wide log level.

Propagate the udev.conf setting to log_set_max_level()
only if udev_new() is called from within udevd or one of its helpers.

Introduce a UDEV_PROPAGATE_LOG macro that we set with -D on
the gcc command line for all udev binaries we build, but not
for any others. The log_set_max_level() call is guarded by an
ifdef check for that macro, so that it only effects the various
udev binaries.

closes: #4525