net: pkt: Fix removal of empty buffers in net_pkt_pull()

If we have removed first net_buf, then we must not restore the
original cursor as that will point to wrong head net_buf.
Add also unit test to check that the packets are removed
properly.
Clarify the documentation that we are removing data from
beginning of the function, also document that the cursor
is reset after this call.

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>