tidy-up waveform option and add `show_contents' function

* report early any error while getting the waveform id
* move all the logic to show contents into show_contents
* log the waveform being used in show_contents
* change order of functions for readability
* remove dead code (epview_opt)
read from stdin if no file name specified

If no file name is specified, read from stdin.  This requires the
new -t option to be set with the document type (i.e. png).  This
cannot be mixed with file names and doesn't support loop and
pause options.  Synchronization options (-s and -p) are still
supported with stdin.

* add `-t' option for document type
* use stdin when no file name specified
* add epdoc_opt.doc_type
* update help message
plpng.c: add support for stdin

Use stdin when file_name is NULL.  The libpng supports this
natively when PNG_STDIO_SUPPORTED is defined (and it normally
is - if not build fails).

* check doc->opt.doc_type is "png" when no file name is given
* do not open or close stdin
ChangeLog v1.9 -> v1.10