Fix #91: Add support for tty nologin, to start /bin/sh immediately

This patch adds a `nologin` flag to the `tty` configuration directive,
making it possible to set up really simple embedded systems with no
login.

Example, put the following line in `/etc/finit.d/getty.conf` to get
a "Please press Enter to ..." prompt before being presented with a
root shell prompt.

    tty [12345] @console noclear nologin

Needless to say, this is not very secure, but can be really useful
for developer setups, during board bringup, or similar.

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

src/helpers.h

@@ -52,6 +52,7 @@ struct fstab *getfsent  (void);
 char   *strip_line      (char *line);
 
 int     getty           (char *tty, speed_t speed, char *term, char *user);
+int     sh              (char *tty);
 
 int     stty            (int fd, speed_t speed);
 speed_t stty_parse_speed(char *baud);
@@ -74,6 +75,7 @@ int     run_interactive (char *cmd, char *fmt, ...);
 int     exec_runtask    (char *cmd, char *args[]);
 pid_t   run_getty       (char *tty, char *baud, char *term,  int noclear, int nowait, struct rlimit rlimit[]);
 pid_t   run_getty2      (char *tty, char *cmd, char *args[], int noclear, int nowait, struct rlimit rlimit[]);
+pid_t   run_sh          (char *tty, int noclear, int nowait, struct rlimit rlimit[]);
 int     run_parts       (char *dir, char *cmd);
 
 static inline void create(char *path, mode_t mode, uid_t uid, gid_t gid)