qrexec: get rid of shell in services using EOF for any signaling

Additional running shell could prevent EOF from being detected.

qubes-rpc/qubes.Filecopy

@@ -1 +1 @@
-/usr/lib/qubes/qfile-unpacker
+exec /usr/lib/qubes/qfile-unpacker

qubes-rpc/qubes.OpenInVM

@@ -1 +1 @@
-/usr/lib/qubes/vm-file-editor
+exec /usr/lib/qubes/vm-file-editor

qubes-rpc/qubes.VMShell

@@ -1 +1 @@
-/bin/bash
+exec /bin/bash