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Change autoconf to test for setmntent() and use that to decide whether
to use getmntent() and setmntent(), since some systems don't have
setmntent() but they do have the mntent.h header file.

Also, remove the includes of mntent.h from e2fsck and mke2fs and other
places where it is not needed.
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>