udev: use LOG_REALM_UDEV in all udev code

Any call to set/query/use the log level in the code with LOG_REALM=LOG_REALM_UDEV
refers to log_max_level[1]. In particular this means that systemd code using
the libudev library uses does not set the log level for log calls done in libudev.

Fixes #4525.

v2:
- also update meson's meson.build