Add default value for length in mb_substr and mb_strcut

[Irker]

In this commit(s) I added "null" default values for length in mb_strcut and mb_substr.
So now we can write:
$result = mb_substr("12345", 2, null, "UTF-8");
And get expected result : "345"

I added this similar to function array_slice (length parametr) php.net/manual/en/function.array-slice.php

I think no great BC there. All exist tests passed.

If you like this idea, I can add tests for my patch.

Yes, I know about mb_internal_encoding.
Yes, I know about discussion about "default" keyword.

[hirokawa]

I think that this patch is valuable, and acceptable.

[lstrojny]

@Irker this fixes a long standing inconvenience, thanks. Could you add a test case?

[php-pulls]

Comment on behalf of lstrojny at php.net:

Merged to PHP-5.4 and master. Thanks for your contribution!