CLEANUP: ncbuf: remove ncb_blk args by value

CLEANUP: ncbuf: remove ncb_blk args by value

ncb_blk structure is used to represent a block of data or a gap in a
non-contiguous buffer. This is used in several functions for ncbuf
implementation. Before this patch, ncb_blk was passed by value, which is
sub-optimal. Replace this by const pointer arguments.

This has the side-effect of suppressing a compiler warning reported in
older GCC version :
  CC      src/http_conv.o
  src/ncbuf.c: In function 'ncb_blk_next':
  src/ncbuf.c:170: warning: 'blk.end' may be used uninitialized in this function

This should be backported up to 2.6.