Fix the -s and -i flags and add support for the "implied_shell"

option.  If the user does not specify a command, sudo will now pass
in the path to the user's shell and set impied_shell=true.  The
plugin can them either check the command normally or return -2 to
cause sudo to print a usage message and exit.

src/sudo.h

@@ -117,6 +117,7 @@ struct user_details {
     const char *cwd;
     const char *tty;
     const char *host;
+    const char *shell;
     GETGROUPS_T *groups;
     int ngroups;
     int ts_cols;
@@ -210,6 +211,9 @@ int exec_setup(struct command_details *details);
 extern int debug_level;
 extern struct plugin_container_list io_plugins;
 
+/* parse_args.c */
+void usage(int) __attribute__((__noreturn__));
+
 #ifndef errno
 extern int errno;
 #endif