Make tlsext_*_{build,needs,parse}() functions static

Make tlsext_*_{build,needs,parse}() functions static

None of these functions are used outside of ssl_tlsext.c. The only reason
they are prototyped in the header is for the use of tlsexttest.c. Rather
than having a big pile of useless copy-paste in the header, we can adapt
the test to avoid using these functions directly.

ok jsing