bio/b_addr.c: omit private hstrerror.

Private hstrerror was introduced to address linking problem on HP-UX,
but truth be told conemporary systems, HP-UX included, wouldn't come
to that call, they would use getaddrinfo and gai_strerror, while
gethostbyname and h_errno are there to serve legacy systems. Since
legacy systems are naturally disappearing breed, we can as well just
let user interpret number.

GH#2816

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>