0.20:
  add errmsg API
  work around broken Linux sendfile API (offset 64-bit but count 32-bit)
  add io_appendfile, io_readwritefile
  support ip6.arpa in addition to ip6.int in dns_name (adds one
    parameter to dns_name6_domain and two constants in dns.h)
  Solaris compatibility for io_passfd and io_receivefd