libnm: normalize "connection.uuid"

For NetworkManager profiles, "connection.uuid" is the identifier of the
profile. It is supposed to be a UUID, however:

- the UUID was not ensured to be all-lower case. We should make sure
  that our UUIDs are in a consistent manner, so that users can rely
  on the format of the string.

- the UUID was never actually interpreted as a UUID. It only was some
  opaque string, that we use as identifier. We had nm_utils_is_uuid()
  which checks that the format is valid, however that did not fully
  validate the format, like it would accept "----7daf444dd78741a59e1ef1b3c8b1c0e8"
  and "549fac10a25f4bcc912d1ae688c2b4987daf444d" (40 hex characters).

Both invalid UUIDs and non-normalized UUID should be normalized. We
don't want to break existing profiles that use such UUIDs, thus we don't
outright reject them. Let's instead mangle them during
54 jobs for th/uuid in 44 minutes and 9 seconds (queued for 18 seconds)