printf(): Save errno earlier.

The manual page says %m is replaced with “the string representation of
the error code stored in the errno variable at the beginning of the
call”. However, we don't actually save errno until fairly late in
__vfprintf(). Make sure it is saved before we do anything that
might perturb errno.

MFC after: 1 week
Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D46718