Disable drivers that need serial port support if libserialport is not found.
Also, disable building various other serial port related code in that case.
The per-driver API calls no longer have a hw_ prefix (e.g. hw_init()
became init() and so on), so drop the 'hw_' from the std versions
for those API callbacks too.
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.