fix IntegerField minimum value

[xbb]

Set the minimum value using PHP_INT_MIN which should be equal to -PHP_INT_MAX - 1

[agh1467]

I found the commit where this was introduced, there is a short conversation there which might add context to the reason why it was that way. https://github.com/opnsense/core/commit/06ff13943b3cddc378005bcd8995c105bb8057a0

[xbb]

I found the commit where this was introduced, there is a short conversation there which might add context to the reason why it was that way. 06ff139

I see, PHP_INT_MIN was introduced in PHP 7.0 and opnsense at that time was using, I think, PHP 5.6

[AdSchellevis]

@xbb thanks!