Description:  this code adds a new output filter to dot/tcldot.
The new filter is called "imap".  It is used just like "ps,"
"ismap," "gif," and the rest of the output filters.

The new filter produces an "imap" file, the new-style
Apache-compatible equivalent of the old-style "ismap" file which
dot already knows how to produce.  In either case, the purpose is
to map regions or points on a gif image to URLs, so that the user
of a web browser can click on the gif image and activate anchors
that invoke other pages.  The "ismap" map file style which was
commonly used with early (NCSA) http servers required an external
mapping mechanism kept in the cgi-bin dir (ismap executable or
user-supplied equivalent), whereas recent Apache servers include
the imap functionality in the server itself.  To configure an
Apache server to use imap files correctly, uncomment this line

	# If you wish to use server-parsed imagemap files, use
	AddHandler imap-file map

in srm.conf.  From then on, you can use files of the form mumble.map
as follows:

     <A HREF="./EER.Database.map">
     <IMG ISMAP SRC="./EER.Database.gif">
     </A>

The .map file will be used as the image map for the associated
gif image.  The new "imapgen" filter for dot produces valid imap
files for use with Apache servers.

In addition to being Apache-compliant, the imap file produced
by imapgen can include clickable edge points as well as node
rectangles.  If the attribute "URL" is defined for an edge, then
the label virtual node will be bound/mapped to that URL, as will
spline points along the edge.  If the special attributes "tailURL"
and/or "headURL" are set, then the head and tail points of the
edge will be bound to those URL, otherwise the head and tail will
default to the main URL.  The older ismapgen code did not
support edge URLs.

[JCE may wish to correct my wording in the description of ismap
 (old-style) files above]