basic: split out dev_t related calls into new devno-util.[ch]

basic: split out dev_t related calls into new devno-util.[ch]

No actual code changes, just splitting out of some dev_t handling
related calls from stat-util.[ch], they are quite a number already, and
deserve their own module now I think.

Also, try to settle on the name "devnum" as the name for the concept,
instead of "devno" or "dev" or "devid". "devnum" is the name exported in
udev APIs, hence probably best to stick to that. (this just renames a
few symbols to "devum", local variables are left untouched, to make the
patch not too invasive)

No actual code changes.