Allow to mixed and specify *.rb and *.mrb in bin/mruby

It is not decides by the extension.
In order to be recognized as a `.mrb` file, the following three points must be satisfied:
- File starts with "RITE"
- At least `sizeof(struct rite_binary_header)` bytes can be read
- `NUL` is included in the first 64 bytes of the file
If these are not met, it is judged as a text file and it is processed as a Ruby script.

The `bin/mruby -b` switch is still available which treats the given file as a `.mrb` file.

New `MRB_API` function:
- `include/mruby/compile.h` and `mrbgems/mruby-compiler/core/parse.y`
  - `mrb_load_detect_file_cxt()` (remove with `MRB_DISABLE_STDIO`)

NOTE:
- Even script files now always open in binary mode for `bin/mruby`.
  The `\r\n` is handled by the `nextc()` function already, so there is no problem even on Windows.
- The `nextc0()` function in `mrbgems/mruby-compiler/core/parse.y` can now specify a string buffer and a file pointer at the same time.
  In this case, get it from the string buffer first.

This patch includes modifies by comment of https://github.com/mruby/mruby/pull/5157.

include/mruby/compile.h

@@ -116,6 +116,7 @@ struct mrb_parser_state {
   mrb_ast_node *cells;
   const char *s, *send;
 #ifndef MRB_DISABLE_STDIO
+  /* If both f and s are non-null, it will be taken preferentially from s until s < send. */
   FILE *f;
 #endif
   mrbc_context *cxt;
@@ -193,6 +194,7 @@ MRB_API mrb_value mrb_load_exec(mrb_state *mrb, struct mrb_parser_state *p, mrbc
 #ifndef MRB_DISABLE_STDIO
 MRB_API mrb_value mrb_load_file(mrb_state*,FILE*);
 MRB_API mrb_value mrb_load_file_cxt(mrb_state*,FILE*, mrbc_context *cxt);
+MRB_API mrb_value mrb_load_detect_file_cxt(mrb_state *mrb, FILE *fp, mrbc_context *c);
 #endif
 MRB_API mrb_value mrb_load_string(mrb_state *mrb, const char *s);
 MRB_API mrb_value mrb_load_nstring(mrb_state *mrb, const char *s, size_t len);