More updates from Bernhard Fischer

More updates from Bernhard Fischer

- flag rrd_resize's old file with RRD_COPY
- cleanup error-handling pathes in rrd_update and fix a few typos in
  comments
- rrd_close(): implement printing mincore results for the rrd if
  DEBUG=2 was defined
- rrd_open(): madvise start addresses need to be page-aligned; implement
  alternative path to the fine-grained (i.e. exact) madvise by flagging
  just the first two pages as needed (see TWO_PAGES). Implement
  alternative path that records the last madvise()ed area to avoid
  redundant calls to madvise() on identical areas (due to
  page-alignment constraints) -- see CHECK_MADVISE_OVERLAPS. Implement
  path for USE_DIRECT_IO.
- configure: add check for O_DIRECT flag to open(2). Add option
  --enable-direct-io. Add _GNU_SOURCE to CFLAGS to silence warnings
  about chroot which is marked LEGACY since SUSv2 and is a non POSIX
  extension. Make checks for posix_fadvise() dependant on
  --disable-mmap, since we do not need fadvise for the mmap case.

git-svn-id: svn://svn.oetiker.ch/rrdtool/trunk/program@1088 a5681a0c-68f1-0310-ab6d-d61299d08faa