MINOR: ncbuf: define various insertion modes

MINOR: ncbuf: define various insertion modes

Define three different ways to proceed insertion. This configures how
overlapping data is treated.
- NCB_ADD_PRESERVE : in this mode, old data are kept during insertion.
- NCB_ADD_OVERWRT : new data will overwrite old ones.
- NCB_ADD_COMPARE : this mode adds a new test in check stage. The
  overlapping old and new data must be identical or else the insertion
  is not conducted. An error NCB_RET_DATA_REJ is used in this case.

The mode is specified with a new argument to ncb_add() function.