Use g_malloc0() for small allocations and assume they always
succeed. Simplify error handling in a few places accordingly.
Don't always sanity-check parameters for non-public (SR_PRIV)
functions, we require the developers to invoke them correctly.
This allows further error handling simplifications.
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.