vncserver: recognize environment variables in configuration files

Since commit c67a5f25 (Start the sessions using xinit), vncserver runs
Xvnc via exec(), instead of via system(). Before that, it was possible
to enable PAM authentication for the user who owned the Xvnc process by
adding this to /etc/tigervnc/vncserver-config-defaults:

    SecurityTypes=TLSPlain
    PlainUsers=$USER

This does not work anymore because with exec() the command line is not
parsed by the user shell.

A simple workaround would be to use "PlainUsers=*" but this creates a
security vulnerability, since it allows any user to have access to the
VNC session, not only the owner of the Xvnc process.

Another workaround is to create a per-user ~/.vnc/config containing

   PlainUsers=<actual-user-name>

but this is inconvenient when there is a large number of VNC users to
configure.

This commits partially restores the old behavior by replacing any $FOO
in a parameter value by the contents of the corresponding environment
variable.

Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=2233204
Signed-off-by: Carlos Santos <casantos@redhat.com>

unix/vncserver/vncserver.in

@@ -285,6 +285,7 @@ sub LoadConfig {
           if ($warnoverride && $config{$k}) {
             print("Warning: $configFile is overriding previously defined '$k' to be '$v'\n");
           }
+          $v =~ s/\$(\w+)/$ENV{$1}/g;
           $config{$k} = $v;
         } elsif ($_ =~ m/^\s*(\S+)/) {
           # We can't reasonably warn on override of toggles (e.g. AlwaysShared)