[tz] include implementations of str{f,p}time{,_l} in tzcode

Hello, I think that it is an omission for those functions not to be included in tzcode. They are tightly bound to the rest of the time code implementation and although standardized, the implementations that the most popular operating systems provide vary in behavior. Yes, I understand that there are portability considerations (locale for example) that need to be handled, but not including them is taking the easy way out. Best, christos