libext2fs: clean up ext2fs_bg_flags_ interfaces

libext2fs: clean up ext2fs_bg_flags_ interfaces

The ext2fs_bg_flag* functions were confusing.

Currently we have this:

void ext2fs_bg_flags_set(ext2_filsys fs, dgrp_t group, __u16 bg_flags);
void ext2fs_bg_flags_clear(ext2_filsys fs, dgrp_t group,__u16 bg_flags);

(_set (unused) sets exactly bg_flags; _clear clears all and ignores bg_flags)

and these, which can twiddle individual bits in bg_flags:

void ext2fs_bg_flag_set(ext2_filsys fs, dgrp_t group, __u16 bg_flag);
void ext2fs_bg_flag_clear(ext2_filsys fs, dgrp_t group, __u16 bg_flag);

A better interface, after the patch below, is just:

ext2fs_bg_flags_zap(fs, group) /* zeros bg_flags */
ext2fs_bg_flags_set(fs, group, flags) /* adds flags to bg_flags */
ext2fs_bg_flags_clear(fs, group, flags) /* clears flags in bg_flags */

and remove the original ext2fs_bg_flags_set / ext2fs_bg_flags_clear.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>