dvbsrc: dvbbasebin: add 'tune' signal

It works the same as the 'tune' property that is used only to signal
the element that it should tune, but it is more natural to be used
as a signal rather than a property.

It is also proxied at the dvbbasebin element
dvbsrc: drop gst_dvbsrc_retry_ioctl for retry macro

LOOP_WHILE_EINTR macro makes it easier to handle
assorted ioctl argp types. Functionality is otherwise
equivalent.
dvbsrc: wrap around known-interruptible ioctls

Also get rid of the asumption that some requests like
FE_SET_TONE or FE_SET_VOLTAGE only error out with -1
dvbsrc: get rid of remaining trailing whitespace
dvbsrc: parsechannels: read DVB-S freq in kHz