scpi-pps: Add shadow driver for HPIB supplies (ID? instead of *IDN?)

scpi-pps: Add shadow driver for HPIB supplies (ID? instead of *IDN?)

HP instruments predating the 488.2 and SCPI standards do not
necessarily have a SCPI-compliant command set. The HP6630A series of
supplies is one such example.

While scpi-pps is flexible enough to accomodate almost any command
syntax given the right profile, it still assumes that "*IDN?" is the
correct question to ask the instrument. Since older HP gear instead
responds to "ID?", this assumption is no longer true.

Thus sr_scpi_get_hw_id() is not appropriate for these instruments, and
the shadow driver added here only replaces that function call, while
reusing the rest of the existing logic. The extra noise is necessary
in order to propagate this through the .scan member of the driver.