udev: net-name-slot - disable by kernel command line switch

The properties will still be set in the udev database, but they will not be used
for setting the interface names. As for the other kernel commandline switches,
we allow it to be prefixed by 'rd.' to only apply in the initrd.
udev: builtin - use RUN rather than IMPORT for loading modules

The 'kmod' builtin, like the 'firmware' and 'uaccess' builtins, does not set
any variables, so don't use IMPORT.

Notice that this changes the behaviour slightly: the processing of subsequent
rules for the event that loads a module will no longer wait for the module
loading to finish. This is not expected to cause any problems, but we should
keep an eye on it.
man: udev - clarify when RUN is executed

Unlike IMPORT and PROGRAM, RUN is not executed inline, but after all the rules of the given event have been processed.