Rework Fformat to avoid integer overflow issues.

Rework Fformat to avoid integer overflow issues.

* editfns.c: Include <float.h> unconditionally, as it's everywhere
now (part of C89).  Include <verify.h>.
(MAX_10_EXP, CONVERTED_BYTE_SIZE): Remove; no longer needed.
(pWIDE, pWIDElen, signed_wide, unsigned_wide): New defns.
(Fformat): Avoid the prepass trying to compute sizes; it was only
approximate and thus did not catch overflow reliably.  Instead, walk
through the format just once, formatting and computing sizes as we go,
checking for integer overflow at every step, and allocating a larger
buffer as needed.  Keep track separately whether the format is
multibyte.  Keep only the most-recently calculated precision, rather
than them all.  Record whether each argument has been converted to
string.  Use EMACS_INT, not int, for byte and char and arg counts.
Support field widths and precisions larger than INT_MAX.  Avoid
sprintf's undefined behavior with conversion specifications such as %#d
and %.0c.  Fix bug with strchr succeeding on '\0' when looking for
flags.  Fix bug with (format "%c" 256.0).  Avoid integer overflow when
formatting out-of-range floating point numbers with int
formats. (Bug#8668)