only spawn one process for the management daemon

In 0088fb4553 I changed the management daemon's startup
script from a symlink to a Python script to a bash script that activated the new virtualenv
and then launched Python. As a result, the init.d script that starts the daemon would
write the pid of bash to the pidfile, and when trying to kill it, it would kill bash but
not the Python process.

Using exec to start Python fixes this problem by making the Python process have the pid
that the init.d script knows about.

fixes #1339

CHANGELOG.md

@@ -6,6 +6,7 @@ In Development
 
 Control Panel:
 
+* The background service for the control panel was not restarting on updates, leaving the old version running. This was broken in v0.26 and is now fixed.
 * Installing your own TLS/SSL certificate had been broken since v0.24 because the new version of openssl became stricter about CSR generation parameters.
 
 v0.26b (January 25, 2018)

setup/management.sh

@@ -100,7 +100,7 @@ rm -f /usr/local/bin/mailinabox-daemon # old path
 cat > $inst_dir/start <<EOF;
 #!/bin/bash
 source $venv/bin/activate
-python `pwd`/management/daemon.py
+exec python `pwd`/management/daemon.py
 EOF
 chmod +x $inst_dir/start
 rm -f /etc/init.d/mailinabox