Negative width arguments now left align the way a minus-width in the
format string aligns. The minus in the format string overrides the sign
of the argument as in C.

Precision behavior is modified to include an error if the argument is
negative.  This differs from a negative precision in a format string
which just terminates the format.

Additional checks for large magnitude widths and precisions are added to
make the runtime behavior (failure, but with different error messages),
more consistent between format string specified width/precision and
argument specified width/precision.

Fixes #11376

Change-Id: I8c7ed21088e9c18128a45d4c487c5ab9fafd13ef
Reviewed-on: https://go-review.googlesource.com/11405Reviewed-by: Rob Pike <r@golang.org>
Run-TryBot: Rob Pike <r@golang.org>