iio: iio_generic_buffer: Cleanup when receiving signals

This will clean (disable buffer/trigger/channels) when doing
something like a CTRL-C. Otherwise restarting generic_buffer requires a
manual echo 0 > buffer/enable

This also cleanup up all the code freeing string buffers at
the end of main. We initialize all pointers to NULL so that cleanup can
all be done under a single error label.

Signed-off-by: Crestez Dan Leonard <leonard.crestez@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>