Run new clients in a new process session

When running a terminal emulator containing a kakoune client,
it used to still have the server tty as controlling termnal,
and so received SIGINT's from the user in it. Termnal emulators
then responded by exiting. This is no longer the case.

src/rc/client.kak

@@ -8,6 +8,5 @@ decl str termcmd %sh{
 
 def new -shell-params %{ nop %sh{
     if (( $# != 0 )); then kakoune_params="-e '$@'"; fi
-    ${kak_opt_termcmd} "kak -c ${kak_session} ${kakoune_params}" < /dev/null >& /dev/null &
-    disown
+    setsid ${kak_opt_termcmd} "kak -c ${kak_session} ${kakoune_params}" < /dev/null >& /dev/null &
 }}