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

Add an FSF copyright notice, since our changes are becoming nontrivial.
Include stdint.h if available, as Autoconf suggests.
(ALIGNBYTES, ALIGN): Remove; no longer needed now that fts_statp
is an array.
(fts_alloc, fts_palloc, fts_sort, fts_load, fts_build):
Use size_t for sizes.
(fts_stat, fts_safe_changedir, fts_debug, fts_read, fts_build,
fts_palloc):
Use bool when appropriate.
(SIZE_MAX, TYPE_SIGNED): New macros.
(fts_read): Use u_short for instructions.
(fts_build): Use ptrdiff_t for levels.  Don't assume file name lengths
fit into int.  Don't assume nlink_t is signed.
(find_matching_ancestor): Don't assume dev, ino fit in int.
(fts_stat): Use function prototype; required for bool arg.
(fts_sort): Detect integer overflow in size calculations.
(fts_alloc): Simplify allocation code, now that fts_statp is an array
and not a pointer.