Allow default inode_size to be specified in mke2fs.conf and document mke2fs -I

Allow default inode_size to be specified in mke2fs.conf and document mke2fs -I

This patch allows "inode_size" to be specified in the mke2fs.conf file,
and always compiles in the "-I" option.  In addition, it disallows
specifying the inode size on rev 0 filesystems, though I don't think
this was much of a danger anyways.

Clean up dead lines in ext2fs.h.

Signed-off-by: Andreas Dilger <adilger@clusterfs.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>