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
bcf2dd0cd2