Cleanup ht's use of positive/strictpositive

Currently, ht.py uses a bad terminology for positive/non-negative
numbers. Per http://en.wikipedia.org/wiki/Positive_number

, this is the
correct terminology:

- A number is positive if it is greater than zero.
- A number is negative if it is less than zero.
- A number is non-negative if it is greater than or equal to zero.
- A number is non-positive if it is less than or equal to zero.

So this patch renames things as follows:

- TPositiveInt            ⇒ TNonNegativeInt
- TStrictPositiveInt      ⇒ TPositiveInt
- TMaybePositiveInt       ⇒ dropped, not used anywhere
- TMaybeStrictPositiveInt ⇒ TMaybePositiveInt
- TPositiveFloat          ⇒ TNonNegativeFloat
- TStrictNegativeInt      ⇒ TNegativeInt

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>