varlinkctl: add protocol upgrade support

The varlink spec supports protocol upgrades and they are very
useful to e.g. transfer binary data directly via varlink. So
far varlinkctl/sd-varlink was not supporting this. This commit
adds support for it in varlinkctl by using the new code in
sd-varlink and the generalized socket-forward code.

shell-completion/bash/varlinkctl

@@ -43,7 +43,7 @@ _varlinkctl() {
     local cur=${COMP_WORDS[COMP_CWORD]} prev=${COMP_WORDS[COMP_CWORD-1]}
     local -A OPTS=(
         [STANDALONE]='-h --help --version --no-pager -q --quiet --no-ask-password
-                      --oneway --collect --more --exec -j -E'
+                      --oneway --collect --more --exec --upgrade -j -E'
         [ARG]='--graceful --timeout --push-fd --json'
     )