units: enable MaxConnectionsPerSocket= for all our Accept=yes units

Let's make sure that user's cannot DoS services for other users so
easily, and enable MaxConnectionsPerSocket= by default for all of them.

Note that this is mostly paranoia for systemd-pcrextend.socket and
systemd-sysext.socket: the socket is only accessible to root anyway,
hence the accounting shouldn#t change anything. But this is just a
safety net, in preparation that we open up some functionality of these
services sooner or later.

units/systemd-coredump.socket

@@ -19,3 +19,4 @@ ListenSequentialPacket=/run/systemd/coredump
 SocketMode=0600
 Accept=yes
 MaxConnections=16
+MaxConnectionsPerSource=8

units/systemd-creds.socket

@@ -18,3 +18,4 @@ ListenStream=/run/systemd/io.systemd.Credentials
 FileDescriptorName=varlink
 SocketMode=0666
 Accept=yes
+MaxConnectionsPerSource=16

units/systemd-pcrextend.socket

@@ -20,6 +20,7 @@ ListenStream=/run/systemd/io.systemd.PCRExtend
 FileDescriptorName=varlink
 SocketMode=0600
 Accept=yes
+MaxConnectionsPerSource=16
 
 [Install]
 WantedBy=sockets.target

units/systemd-sysext.socket

@@ -20,6 +20,7 @@ ListenStream=/run/systemd/io.systemd.sysext
 FileDescriptorName=varlink
 SocketMode=0600
 Accept=yes
+MaxConnectionsPerSource=16
 
 [Install]
 WantedBy=sockets.target