dnsfilter reads a series of lines from stdin, converts an IP
address to a host name at the beginning of each line, and prints
the results to stdout. If a line does not begin with an IP
address, dnsfilter leaves the line alone. If an IP address does
not have a host name listed in DNS, dnsfilter leaves the line
alone. If an IP address has a host name listed in DNS, dnsfilter
inserts an equals sign and the host name before the first space
or tab in the line. If a DNS lookup fails temporarily, dnsfilter
inserts a colon and a dash-separated error message before the
first space or tab in the line. While dnsfilter is looking up an
address in DNS, it reads ahead in the input and looks for more
addresses to look up in parallel. opts is a series of getopt-
style options: