e2fsck: use different bitmap types as appropriate

e2fsck: use different bitmap types as appropriate

Now that we have multiple backend implementations of the bitmap code,
this commit teaches e2fsck to use either the most appropriate backend
for each use case.

Since we don't know for sure if we will get it all right, the default
choices can be overridden via e2fsck.conf.  The various definitions
are shown here, with the current defaults (which may change as we add
more bitmap implementations and as learn what works better).

; EXT2FS_BAMP64_BITARRAY is 1
; EXT2FS_BMAP64_RBTREE is 2
; EXT2FS_BMAP64_AUTODIR is 3
[bitmaps]
	inode_used_map = 2	; pass1
	inode_dir_map = 3	; pass1
	inode_reg_map = 2	; pass1
	block_found_map = 2	; pass1
	inode_bad_map = 2	; pass1
	inode_imagic_map = 2	; pass1
	block_dup_map = 2	; pass1
	block_ea_map = 2	; pass1
	inode_link_info = 2	; pass1
	inode_dup_map = 2	; pass1b
	inode_done_map = 3	; pass3
	inode_loop_detect = 3	; pass3
	fs_bitmaps = 2

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