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
nm_connection_normalize().
54 jobs for th/uuid in 44 minutes and 9 seconds (queued for 18 seconds)