Fix daemontools run script for dash shell

Fixes #374

contrib/daemontools/run

@@ -1,3 +1,5 @@
 #!/bin/sh
 echo "Launching ElectrumX server..."
-exec 2>&1 envdir ./env /bin/sh -c 'setuidgid $USERNAME python3 $ELECTRUMX'
+USERNAME=$(envdir ./env printenv USERNAME)
+ELECTRUMX=$(envdir ./env printenv ELECTRUMX)
+ulimit -n 4000 && exec 2>&1 envdir ./env envuidgid $USERNAME python3 $ELECTRUMX