Merge the ext4 support for imagezip into the extfs code.  There's no
real reason to keep it separate, since ext4 is backward-compatible
with ext3 and ext2.

All of the macro definitions in extfs.c were changed from EXT2_* or
EXT3_* to EXT4_*.  The primary reason for this is that while ext4 is
backward compatible, the data structures have been extended and some
of the definitions needed to change to handle them.  Some things that
were constant before (block group descriptor sizes, for example) are
now dynamic and must be calculated from other fields in the
superblock.

Since imagezip and frisbee don't support 64-bit block numbers, ext4
filesystems that are larger than 2 TB (assuming 512-byte sectors) are
not supported.  These can be detected by examining the
feature_incompat field of the superblock.  Imagezip will fail if the
user tries to compress one of these filesystems.