logging: add special logging level "KEEP"

logging: add special logging level "KEEP"

Without this, the user cannot configure only certain logging domains
without touching them all.

E.g.

  # nmcli general logging level DEBUG domains PLATFORM

will disable all non-PLATFORM domains.
Well, the user can do:

  # nmcli general logging level INFO domains PLATFORM:DEBUG
  # nmcli general logging level DEBUG domains ALL:INFO,PLATFORM

but in this case all non-PLATFORM domains are reset explicitly.

Now the user can:

  # nmcli general logging level KEEP domains PLATFORM:DEBUG
  # nmcli general logging level DEBUG domains ALL:KEEP,PLATFORM

which will only change the platform domain.