Use exec to ease systemd handling the tinyproxy process

From Marek's comments

misc/tinyproxy-wrapper

@@ -2,10 +2,10 @@
 
 if [ -e /usr/sbin/tinyproxy ]; then
     echo "Found tinyproxy at /usr/sbin/tinyproxy"
-    /usr/sbin/tinyproxy "$@"
+    exec /usr/sbin/tinyproxy "$@"
 elif [ -e /usr/bin/tinyproxy ]; then
     echo "Found tinyproxy at /usr/bin/tinyproxy"
-    /usr/bin/tinyproxy "$@"
+    exec /usr/bin/tinyproxy "$@"
 else
     echo "tinyproxy not found"
     exit 1

vm-systemd/qubes-updates-proxy.service

@@ -5,7 +5,7 @@ After=qubes-iptables.service
 
 [Service]
 ExecStartPre=/usr/lib/qubes/iptables-updates-proxy start
-ExecStart=/bin/bash -c '/usr/lib/qubes/tinyproxy-wrapper -d -c /etc/tinyproxy/tinyproxy-updates.conf'
+ExecStart=/usr/lib/qubes/tinyproxy-wrapper -d -c /etc/tinyproxy/tinyproxy-updates.conf
 ExecStopPost=/usr/lib/qubes/iptables-updates-proxy stop
 Restart=on-failure
 RestartSec=5s