libgo: use Solaris sendfile lib, fix Solaris _in6_addr_t fields

libgo: use Solaris sendfile lib, fix Solaris _in6_addr_t fields

    Before Solaris 12 the sendfile function is found in -lsendfile, so look
    for it there.

    Solaris 12 adds some more types that use _in6_addr_t, that need to be
    edited in mksysinfo.

    Patch by Rainer Orth.

    Reviewed-on: https://go-review.googlesource.com/16779

From-SVN: r230132