Add an FSF copyright notice, since our changes are becoming nontrivial.

Add an FSF copyright notice, since our changes are becoming nontrivial.
Include stddef.h, for ptrdiff_t.
(FTS.fts_nitems): Now size_t, not int, for hosts that allow more
than INT_MAX entries in a directory.
(FTS_ROOTPARENTLEVEL): Parenthesize properly.
(FTSENT.fts_level): Now ptrdiff_t, not int, to allow recursing more
than INT_MAX levels deep on 64-bit hosts.
(FTSENT.fts_namelen): Now size_t, not u_short, to support hosts like
the Hurd that don't have arbitrary limits on directory entry lengths.
(FTSENT.fts_statp): Now an array, not a pointer, so that we don't
have to play unportable games with pointer arithmetic.  Keep it array
for the benefit of user code that assumes it is a pointer.