libsigrok-internal.h: add helper macro to read 8 bits integer
matching the 16 and 32 bits one
libsigrok-internal.h: endian neutral helper macro to write 8/16/32 bits integer to unaligned memory
scpi: add a struct drv_context parameter to scpi_dev_inst_new()
scpi: add a generic scan API and implement it in usbtmc and serial transport

note that sr_usb_find_serial() is copied from the hameg-hmo driver
rigol-ds: use the new scpi scan API
hameg-hmo: use the new scpi scan API
scpi: add a libusb based implementation of usbtmc
usb: remove unused sr_usb_find_usbtmc()