Merge in changes from libc's misc/error.c, in preparation

Merge in changes from libc's misc/error.c, in preparation
for the merge of gnulib's changes back into libc.

(_): Define only if not already defined.
Move definition to follow all #include directives.
Include unlocked-io.h only if !_LIBC.
[_LIBC]: Include <libio/libioP.h>.
[USE_IN_LIBIO]: Include <libio/iolibio.h>
(fflush): Tweak definition to use INTUSE.
(putc): Define.