e2fsck: add a developer-only extended option: clear_all_uninit_bits

e2fsck: add a developer-only extended option: clear_all_uninit_bits

This option clears the uninitialized bit on all extents of all inodes.
Note that this can end up exposing uninitialized data to userspace.
It should only used in very specialized situations.

This option is only enabled via a new configure flag,
--enable-developer-features.  It should *not* be enabled by
distributions, as it enables features thare only designed for use by
ext4 developers.  These features have no documentation in the man
page, or regression tests, and if it breaks, you get to keep both
pieces.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>