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

[Christos Zoulas]

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