fmt: improve handling of zero padding

fmt: improve handling of zero padding

Simplify the handling of zero padding in fmt_integer and
fmt_float to not require any adjustment of the format flags.

Note that f.zero can only be true when padding to the left
and f.wid is always greater than or equal to 0.

Change-Id: I204b57d103c0eac13d86995992f2b26209196925
Reviewed-on: https://go-review.googlesource.com/21185
Run-TryBot: Rob Pike <r@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>