scpi-dmm: Implement support for Agilent 34405A, prepare others

Implement the scpi-dmm driver in such a generic way that it could work
with several protocol variants and with differing models which happen to
use any of these protocol variants. Prepare a list of supported models
with their respective SCPI command set, set of DMM functions and their
precision.

Add support for Agilent 34405A. The ten functions of this device got
tested and are operational, in continuous mode as well as with sample
count or capture time limits. The driver can query the current meter's
function, can change the function, and can run acquisitions in either
the current mode or with a user specified function selection. There is
some potential for improvement: AUTO/MIN/MAX/HOLD indicators are not
supported by this implementation.