This is a small helper function which sends the SR_DF_HEADER packet that
drivers usually emit in their hw_dev_acquisition_start() API callback.
It simplifies and shortens the hw_dev_acquisition_start() functions
quite a bit.
It also simplifies the input modules which send an SR_DF_HEADER packet, too.
This patch also automatically removes some unneeded malloc/free in some
drivers for the 'packet' and 'header' structs used for SR_DF_HEADER.
Check the relevant arguments for != NULL before calling the actual
driver-specific function, so that the driver can safely assume those
arguments are non-NULL. This removes the need to duplicate these
checks in every driver.
Also, do some minor whitespace and consistency improvements.
Sending an SR_DF_META packet at the start of every stream is not
mandatory; the frontend should ask for what it needs, and any extra
information the driver wants to send will come in due time.
Currently hw_info_get() can receive requests for entries (info_id) that
the specific driver doesn't support. That is (right now) a valid
use-case and not an error (might change later, though).
Thus, for now, don't output messages for such requests at all (certainly
not as sr_err() where they show up in e.g. sigrok-cli output per default).