CA-86568: Kill only the vncterm parent process

Do not kill the process group.

vncterm will now catch SIGTERM, pass it on to its child process,
wait for it to exit, clean up, and finally exit itself.

Signed-off-by: Miëtek Bak <mietek.bak@citrix.com>
Signed-off-by: David Scott <dave.scott@eu.citrix.com>