build: Use pkgconfig for libgpib.
scpi/gpib: Rename backend to scpi_libgpib.

libgpib is the userspace component to linux-gpib's kernel modules that
implement low-level interface drivers.

When libsigrok gets userspace GPIB interface drivers, that backend will
be the "official" scpi_gpib.
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.