posix_spawn_file_actions_addopen needs to copy the path argument (BZ 17048)

POSIX requires that we make a copy, so we allocate a new string
and free it in posix_spawn_file_actions_destroy.

Reported by David Reid, Alex Gaynor, and Glyph Lefkowitz.  This bug
may have security implications.

posix/spawn_int.h

@@ -22,7 +22,7 @@ struct __spawn_action
     struct
     {
       int fd;
-      const char *path;
+      char *path;
       int oflag;
       mode_t mode;
     } open_action;