stdio: Implement %#m for vfprintf and related functions

%#m prints errno as an error constant if one is available, or
a decimal number as a fallback.  This intends to address the gap
that strerrorname_np does not work well with printf for unknown
error codes due to its NULL return values in those cases.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>