Support bit flags for `IO.open`

In top-level, `mid` is `NULL`. We used to ignore 'mid` update for `NULL`.

Delete duplicated `Array#delete_if`.

This bug was introduced in 694089fa to address #4832

Issue 19902: mruby:mruby_fuzzer: Stack-buffer-overflow in mrb_str_len_to_dbl

This is reported by oss-fuzz:
Issue 19886: mruby:mruby_fuzzer: Potential-null-reference in setup_numparams

Merge branch 'file-size-truncate' of https://github.com/dearblue/mruby into dearblue-file-size-truncate

The basic idea of this change is from @dearblue.

Note: the arguments of `mrb_str_pool()` have changed, but the function
is provided for internal use (No `MRB_API`). So basically you don't have
to worry about the change.

Fix ainfo with keyword arguments; fix #4921

Call `va_end()` before return

The behavior when returning from a function without `va_end()` is
undefined.

Add `mrb_num_args_error()` for "wrong number of arguments" error

Revert "SHARED string is not required when sharing POOL string" (75949836)

Because literal pool may be released by GC.

#### Example:

  ```ruby
  s1 = eval('"abcdefghijklmnopqrstuvwxyz01"')
  GC.start
  p s1  #=> "\x00\x00\x00\x00\x00\x00\x00\x90\x00\x00\x00\x00\x00\x00\x00\x90\x03\x00stuvwxyz01"
  ```

Integrate `i` and `arg_i` in `mrb_get_args()`

Do not include `stdint.h` before `mruby.h`; ref #4750

The behavior of these two variables is the same.

Silence GCC warning in `time.c` on Travis CI

Silence the following warnings:

  ```
  /mruby/mrbgems/mruby-time/src/time.c:260:55: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
    if ((MRB_INT_MAX > MRB_TIME_MAX && i > 0 && i > MRB_TIME_MAX) ||
                                                  ^
  ```