add wrapper headers, with warnings, for various incorrect names under sys

also add a warning to the existing sys/poll.h. the warning is absent
from sys/dir.h because it is actually providing a slightly different
API to the program, and thus just replacing the #include directive is
not a valid fix to programs using this one.

include/sys/dir.h

+#include <dirent.h>
+#define direct dirent

include/sys/errno.h

+#warning redirecting incorrect #include <sys/errno.h> to <errno.h>
+#include <errno.h>

include/sys/fcntl.h

+#warning redirecting incorrect #include <sys/fcntl.h> to <fcntl.h>
+#include <fcntl.h>

include/sys/poll.h

+#warning redirecting incorrect #include <sys/poll.h> to <poll.h>
 #include <poll.h>

include/sys/signal.h

+#warning redirecting incorrect #include <sys/signal.h> to <signal.h>
+#include <signal.h>

include/sys/termios.h

+#warning redirecting incorrect #include <sys/termios.h> to <termios.h>
+#include <termios.h>