split internal.h into files

One day, I could not resist the way it was written.  I finally started
to make the code clean.  This changeset is the beginning of a series of
housekeeping commits.  It is a simple refactoring; split internal.h into
files, so that we can divide and concur in the upcoming commits.  No
lines of codes are either added or removed, except the obvious file
headers/footers.  The generated binary is identical to the one before.

internal/io.h

@@ -0,0 +1,31 @@
+#ifndef INTERNAL_IO_H /* -*- C -*- */
+#define INTERNAL_IO_H
+/**
+ * @file
+ * @brief      Internal header for IO.
+ * @author     \@shyouhei
+ * @copyright  This  file  is   a  part  of  the   programming  language  Ruby.
+ *             Permission  is hereby  granted,  to  either redistribute  and/or
+ *             modify this file, provided that  the conditions mentioned in the
+ *             file COPYING are met.  Consult the file for details.
+ */
+
+/* io.c */
+void ruby_set_inplace_mode(const char *);
+void rb_stdio_set_default_encoding(void);
+VALUE rb_io_flush_raw(VALUE, int);
+#ifdef RUBY_IO_H
+size_t rb_io_memsize(const rb_io_t *);
+#endif
+int rb_stderr_tty_p(void);
+void rb_io_fptr_finalize_internal(void *ptr);
+#define rb_io_fptr_finalize rb_io_fptr_finalize_internal
+
+RUBY_SYMBOL_EXPORT_BEGIN
+/* io.c (export) */
+void rb_maygvl_fd_fix_cloexec(int fd);
+int rb_gc_for_fd(int err);
+void rb_write_error_str(VALUE mesg);
+RUBY_SYMBOL_EXPORT_END
+
+#endif /* INTERNAL_IO_H */