prevent psignal/psiginfo from clobbering stderr orientation, errno

these functions are specified to write to stderr but not set its
orientation, presumably so that they can be used in programs operating
stderr in wide mode. also, they are not allowed to clobber errno on
success. save and restore to meet the requirement.

psiginfo is reduced to a think wrapper around psignal, since it
already behaved the same. if we want to add more detailed siginfo
printing at some point this will need refactoring.

src/signal/psiginfo.c

-#include <stdio.h>
-#include <string.h>
 #include <signal.h>
 
 void psiginfo(const siginfo_t *si, const char *msg)
 {
-	char *s = strsignal(si->si_signo);
-	if (msg) fprintf(stderr, "%s: %s\n", msg, s);
-	else fprintf(stderr, "%s\n", s);
+	psignal(si->si_signo, msg);
 }

src/signal/psignal.c

-#include <stdio.h>
+#include "stdio_impl.h"
 #include <string.h>
 #include <signal.h>
+#include <errno.h>
 
 void psignal(int sig, const char *msg)
 {
+	FILE *f = stderr;
 	char *s = strsignal(sig);
-	if (msg) fprintf(stderr, "%s: %s\n", msg, s);
-	else fprintf(stderr, "%s\n", s);
+
+	FLOCK(f);
+
+	/* Save stderr's orientation and encoding rule, since psignal is not
+	 * permitted to change them. Save errno and restore it if there is no
+	 * error since fprintf might change it even on success but psignal is
+	 * not permitted to do so. */
+	void *old_locale = f->locale;
+	int old_mode = f->mode;
+	int old_errno = errno;
+
+	if (fprintf(f, "%s%s%s\n", msg?msg:"", msg?": ":"", s)>=0)
+		errno = old_errno;
+	f->mode = old_mode;
+	f->locale = old_locale;
+
+	FUNLOCK(f);
 }