Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

The argument corresponding to %zn is supposed to be an ssize_t *, not
a size_t *, although the distinction is moot in practice.

Correctly handle malloc() failure. While here, reduce the code size a
bit by removing some calls to the inline function addtype().

Reduce the level of duplication between vfprintf() and vfwprintf()
by moving the positional argument handling code to a new file,
printf-pos.c, and moving common definitions to printflocal.h.
No functional change intended.