daemon: move WINDOWPATH setting up top of get_welcome_environment
    
    At the top of the function we loop through "optional" environment
    variables and set them in the welcome environment only if they are
    already set in the slave environment.
    
    After commit 62c19ec5d67bf3f4279607012d04f2ef948a63d5 WINDOWPATH
    is optional now as well.
    
    This commit consolidates the WINDOWPATH setting code with the other
    optional environment variable code at the top.