net: buf: Add API for saving & restoring parsing state

Occasionally it may be useful to pass a buffer to a subroutine and
have the routine do parsing of the buffer. However, since there isn't
necessarily a guarantee of how the subroutine performs the parsing it
may be necessary to restore to a well known state after the routine
returns.

This patch adds a simple struct for storing the parsing state as well
as two new functions to save and restore the state.

Change-Id: If9153ff9997021c76243ea9ebff13dfe94c45faa
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>