mke2fs: Disentangle lazy_bg from uninit_bg's lazy inode table initialization

mke2fs: Disentangle lazy_bg from uninit_bg's lazy inode table initialization

Instead of using -O lazy_bg,uninit_bg as the way of requesting that
the inode table be lazy unitialized, use the parameter
lazy_itable_init, which can either be set via mke2fs's -E option, or
via /etc/mke2fs.conf.

Also fix some random problems in mke2fs's man page, including
documenting the extent feature, which had been missing.

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