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

[Random832]

Matt Johnson <mj1856 at hotmail.com> writes:
> Besides, I don't think tzcode is intended to be a general-purpose date
> and time library, is it? If so, strf/p is just the tip of the iceberg.

I'm not sure what exactly the library sections tzcode are meant to be,
these days. My impression is that it was originally a drop-in
replacement for libc functions on Unix systems from before these
features standardized or were widely implemented. That would explain why
it _does_ ship with a version of strftime.