Fix 'kill -INFO' on systems that support SIGINFO (#59)

src/cmd/ksh93/data/signals.c:
- SIGINFO was absent from the table of signals, which caused
  commands like 'kill -INFO $$' to fail even on platforms with
  SIGINFO (such as macOS and FreeBSD). Fix that by adding
  it to the signal table.

src/cmd/ksh93/tests/signal.sh:
- Add a regression tests for using SIGINFO with the kill builtin.
  The test will only be run if the external kill command supports
  SIGINFO.

src/cmd/ksh93/data/signals.c

@@ -107,6 +107,9 @@ const struct shtable2 shtab_signals[] =
 #else
 	"INT",		VAL(SIGINT,SH_SIGINTERACTIVE),			"",
 #endif /* JOBS */
+#ifdef SIGINFO
+	"INFO",		VAL(SIGINFO,SH_SIGIGNORE),			S("Information request"),
+#endif	/* SIGINFO */
 #ifdef SIGIO
 	"IO",		VAL(SIGIO,SH_SIGDONE),				S("IO signal"),
 #endif	/* SIGIO */