qrexec: in agent, handle CONNECT_EXISTING the way convenient for client
qrexec: in agent, handle CONNECT_EXISTING the way convenient for client

QubesOS/qubes-core-admin,marmarek/qubes-core-admin,woju/qubes-core-admin,QubesOS/qubes-core-admin,woju/qubes-core-admin,marmarek/qubes-core-admin,woju/qubes-core-admin,QubesOS/qubes-core-admin,marmarek/qubes-core-admin,woju/qubes-core-admin