desktop, sysui: let --convert-to imply --headless

It's easy to forget --headless before --convert-to, and it's hard to
image that would be intentional ever.

Change-Id: I18b85837239d9450743f05d94bb2673c7d13246d

desktop/source/app/cmdlineargs.cxx

@@ -465,6 +465,8 @@ void CommandLineArgs::ParseCommandLine_Impl( Supplier& supplier )
                 bOpenEvent = false;
                 bConversionEvent = true;
                 bConversionParamsEvent = true;
+                // It doesn't make sense to use convert-to without headless.
+                setHeadless();
             }
             else if ( oArg == "print-to-file" )
             {

desktop/source/app/cmdlinehelp.cxx

@@ -79,7 +79,7 @@ namespace desktop
         "don't show startup screen.\n"\
         "don't check for remote instances using the installation\n"\
         "don't start with an empty document\n"\
-        "like invisible but no userinteraction at all.\n"\
+        "like invisible but no user interaction at all (implies --headless).\n"\
         "show this message and exit.\n"\
         "display the version information.\n"\
         "create new text document.\n"\

sysui/desktop/man/libreoffice.1

@@ -63,7 +63,7 @@ Starts with a new Calc document.
 \fB\-\-convert\-to\fR output_file_extension[:output_filter_name] [\-\-outdir output_dir] \fIfile\fR...
 Batch converts files.
 If \fI\-\-outdir\fR is not specified then the current working directory is used as the output directory
-for the converted files.
+for the converted files. It implies \fI\-\-headless\fR.
 
 Examples: