Avoid interleaving stderr in a few cases

Avoid interleaving stderr in a few cases

* src/sysdep.c (buferr): New static var.
(init_standard_fds) [_PC_PIPE_BUF]: Initialize it.
(errstream, errputc, verrprintf, errwrite): New functions.
(close_output_streams): Check buferr status too.
* src/xdisp.c: Include sysstdio.h instead of stdio.h.
(message_to_stderr, vmessage): Use the new functions
to avoid interleaving stderr.