Allow pipe + redirect in task/run stanzas, exec() in /bin/sh

This patch extends the capabilities of task and run .conf stanzas by
enabling pipes and redirects.

Example:

    task [s] echo "foo" | cat >/tmp/bar
    run  [s] echo "$HOME" >/tmp/secret

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>

src/helpers.h

@@ -63,6 +63,7 @@ void    set_hostname    (char **hostname);
 int     complete        (char *cmd, int pid);
 int     run             (char *cmd);
 int     run_interactive (char *cmd, char *fmt, ...);
+int     exec_runtask    (char *cmd, char *args[]);
 pid_t   run_getty       (char *tty, char *speed, char *term, int noclear, int nowait, int console);
 pid_t   run_getty2      (char *tty, char *cmd, char *args[], int noclear, int nowait, int console);
 int     run_parts       (char *dir, char *cmd);