scpi-pps: Initial driver skeleton.
Revamp PPS-related config keys.
scpi-pps: Add basic cross-vendor PPS functionality.
scpi-pps: Add support for Rigol DP832.
scpi-pps: Generalize vendor name cleanup.
scpi-pps: Use regex to match model names.
scpi-pps: Add support for Rigol DP831A/DP832A.
scpi-pps: Simplify SCPI command handling.
Add private storage pointer to struct sr_channel.
scpi-pps: Create one channel per MQ/output combination.

This allows frontends to trivially select which MQs on which channels
they want to see; others won't even be fetched.
Add support for HP 6632B.
rigol-ds: replace some magic numbers by appropriate constant or variable

This fixes bug #406.
C++: Fix recursion loop in PacketPayload::get_shared_pointer().
C++: Make Driver inherit Configurable.
C++: Add Configurable::config_keys() method.
C++: Add Configurable::config_check() method.
C++: Preserve original channel ordering.
C++: Add SessionDevice class for devices owned by loaded sessions.
C++: Whitespace fix.
configure.ac: Whitespace, cosmetics, sorting, etc.