mke2fs: Fail if the requested file system type is not defined in mke2fs.conf

mke2fs: Fail if the requested file system type is not defined in mke2fs.conf

If the user passes a file system type which is not defined in
mke2fs.conf (i.e., mke2fs -t xfs ...) change mke2fs so that it prints
a warning and aborts the run.  (There is an exception for ext2, since
that file system does not need a special definition in the fs_types
section of the /etc/mke2fs.conf file.)

In addition, print a warning if there are usage types (specified using
the -T option) which are not defined in /etc/mke2fs.conf.

Addresses-Debian-Bug: #594609

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