FIX: DSHOW "none" device open (#2201). Some cleaning

FIX: DSHOW "none" device open (#2201). Some cleaning

-Fix dshow "none" deivice be recognized as none
 Note that another patch concerning var_optionParse string
 dequoting should be applied for this to be fixed
 ( the concept is not to provide quoted strings to dshow options )
 -Account for the case when BOTH devices could not be opened
   (file an error, instead of seeming to play)
 -Cleaned a little
 -Added dbg and error msgs
 -Added coments

Rev 3

Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>