libext2fs: add bitmap statistics

libext2fs: add bitmap statistics

This feature is especially useful for better understanding how e2fsprogs
tools (mainly e2fsck) treats bitmaps and what bitmap backend can be most
suitable for particular bitmap. Backend itself (if implemented) can
provide statistics of its own as well.

[ Changed to provide basic statistics when enabled with the
  E2FSPROGS_BITMAPS_STATS environment variable -- tytso]

Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>