stat-util: add new APIs device_path_make_{major_minor|canonical}() and device_path_pa...

stat-util: add new APIs device_path_make_{major_minor|canonical}() and device_path_parse_major_minor()

device_path_make_{major_minor|canonical)  generate device node paths
given a mode_t and a dev_t. We have similar code all over the place,
let's unify this in one place. The former will generate a "/dev/char/"
or "/dev/block" path, and never go to disk. The latter then goes to disk
and resolves that path to the actual path of the device node.

device_path_parse_major_minor() reverses device_path_make_major_minor(),
also withozut going to disk.

We have similar code doing something like this at various places, let's
unify this in a single set of functions. This also allows us to teach
them special tricks, for example handling of the
/run/systemd/inaccessible/{blk|chr} device nodes, which we use for
masking device nodes, and which do not exist in /dev/char/* and
/dev/block/*