Change "reverse_lookup_name" functions to refer to "PTR_name"s

Change "reverse_lookup_name" functions to refer to "PTR_name"s

Under the new convention, having a tor_addr.*lookup function that
doesn't do hostname resolution is too close for comfort.

I used this script here, and have made no other changes.

  s/tor_addr_parse_reverse_lookup_name/tor_addr_parse_PTR_name/g;
  s/tor_addr_to_reverse_lookup_name/tor_addr_to_PTR_name/g;