Enhance vlib_buffer_clone to change headroom space for packet

Add function vlib_buffer_clone_at_offset() so that the cloned
packets' 1st buffer will have the original packet header copied
at the specified offset (instead of current_data). This can be
used to increase headroom in the cloned packet to allow large
header encaps in case the stdandard 128B predata is not enough.
The original vlib_buffer_clone() still works the same as before.

Change-Id: I3b50c8ad7e3952568bc141710567c99a3dacddce
Signed-off-by: John Lo <loj@cisco.com>