Add an -icon option to set the screen window icon in windowed mode

Allow cygwin paths in an icon-specification

Update man pages and system.XWinrc appropriately

Also, log an error if the icon specified for TRAYICON cannot be loaded

Also, fix a bug in appending a '\' to IconDirectory only if it doesn't
already end with one, which was fortunately benign.

Note: LoadImageComma would be simpler if we just stated that XWinrc
paths are Cygwin paths on Cygwin, Windows paths on MinGW, but that could
break existing .XWinrc files

Note: Given that we can specify paths in an icon-specifier, I'm not sure
what IconDirectory wins us.

v2:
Fix formatting problems in man page additions

v3:
Fix some more s/_/@/g in man pages