serial-dmm, metex14: add support for multiple channels per DMM

serial-dmm, metex14: add support for multiple channels per DMM

Optionally create multiple analog channels in serial-dmm's scan()
routine. Allow the meters' parse routines to fill in more than one
analog value from the inspection of a single packet.

Use "large" (4 times 14 bytes) packets for the Metex M-3860M and the
PeakTech 4390A meters, and have those large packets parsed by wrapping
the routines for regular 14-byte Metex packets, and sending four values
to the session bus after reception of one large packet.

Thanks to Frank Stettner <redacted> for testing and
fixing the initial implementation of this extension.