units: let's establish the coredump socket before writting core_pattern sysctl

It's a bit nicer if we only write the sysctl core_pattern once the
coredump socket is established, since it's the backend for the handler.

Given the systemd-coredump.socket basically has no dependencies that run
before it this should not really make things slower or so, it just
removes the tiny window where core pattern is in effect that wants to
connect to the backend socket but cannot.

The status quo isn't terrible, and not too different in effect: either
way, until the socket unit is up we won't process coredumps. It's mostly
what kind of behaviour you get then: an error due to /bin/false being
invoked, or an error because systemd-coredump can't connect to its
socket. After this patch we'll exclusively see the former.

units/systemd-coredump.socket

@@ -11,7 +11,7 @@
 Description=Process Core Dump Socket
 Documentation=man:systemd-coredump(8)
 DefaultDependencies=no
-Before=shutdown.target
+Before=shutdown.target systemd-sysctl.service
 Conflicts=shutdown.target
 
 [Socket]