Changed list_directory() somewhat. It is now only called when there

Changed list_directory() somewhat.  It is now only called when there
is no index.htm[l] file, and when it is called, it also spits out the
headers.  When an index.htm[l] file is present, the regular (file
access) path is followed.  Also, when the guessed content-type matches
text/*, open the file in text mode; otherwise in binary mode.