basic: split unit-name.[ch] into two (#7065)

It always bothered me a bit that unit-name.[ch] contains so many
definitions that aren't really have much to do with unit nameing, for
example all the unit state definitions.

With this patch unit-name.[ch] is split into two: the file now contains
only the unit naming related operations, and everything else is split
out into a new set of files unit-def.[ch]. That's mostly unit state
stuff as well as dbus path and interface name operations.

No functional changes. This just moves code around.

(Note as both .c files include each other's headers this doesn't make
the build simpler or anything. All it does is make the C files a bit
shorter, and medicate my pretend OCD)