implement psignal and psiginfo

src/signal/psiginfo.c

+#include <stdio.h>
+#include <string.h>
+#include <signal.h>
+
+void psiginfo(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);
+}

src/signal/psignal.c

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