libc: add link_ntoa_r()

this is a re-entrant version of link_ntoa. use an in-out parameter for
the buffer size, so the user requires at most two calls to determine the
needed size.

reimplement link_ntoa using link_ntoa_r with a static buffer.

Reviewed by: des
Approved by: des (mentor)
Differential Revision: https://reviews.freebsd.org/D50202