units: allow disabling systemd-coredumpd.service

The new unit is nice, but it might not be appropriate in all scenarios. It
requires a new kernel, which might be known to not be available, and also
changes sysctl kernel.core_pattern at runtime, which might not be desired.
Replace static enablement by a normal [Install] section and add the unit
to presets to enable it by default.

The awkward instructions in the man page are simplified and adjusted.

NEWS

@@ -368,7 +368,7 @@ CHANGES WITH 262 in spe:
           kernel.core_pattern. Older kernels continue to use the per-coredump
           systemd-coredump@.service path. Changes to coredump.conf require
           reloading systemd-coredumpd.service, and deployments that set a
-          custom kernel.core_pattern should mask or stop that service.
+          custom kernel.core_pattern should stop and disable that service.
 
         * The coredumpctl dump and debug commands now honor --json= and include
           the selected coredump's metadata in the requested JSON format.

man/systemd-coredump.xml

@@ -328,12 +328,8 @@
     <filename>50-coredump.conf</filename> alone does <emphasis>not</emphasis> override the default
     <varname>kernel.core_pattern</varname> on such systems, since <filename>60-coredump.conf</filename> will
     still take effect. To set a custom <varname>kernel.core_pattern</varname> of your own on a system where
-    <filename>systemd-coredumpd.service</filename> is available, you must additionally stop (if currently
-    running) and mask <filename>systemd-coredumpd.service</filename>:
-    <programlisting>systemctl mask --now systemd-coredumpd.service</programlisting>
-    Masking <filename>systemd-coredumpd.service</filename> also prevents
-    <filename>systemd-coredump-register.service</filename>, which is bound to it, from running and writing
-    <filename>60-coredump.conf</filename>.</para>
+    <filename>systemd-coredumpd.service</filename> is active, stop and disable it:
+    <programlisting>systemctl disable --now systemd-coredumpd.service</programlisting></para>
 
     <para>In order to be used in the <option>--backtrace</option> mode, an appropriate backtrace
     handler must be installed on the sender side. For example, in case of

presets/90-systemd.preset

@@ -22,6 +22,7 @@ enable getty@.service
 enable systemd-boot-clear-sysfail.service
 enable systemd-boot-update.service
 enable systemd-confext.service
+enable systemd-coredumpd.service
 enable systemd-homed.service
 enable systemd-homed-activate.service
 enable systemd-journalctl-metrics.socket

units/meson.build

@@ -349,7 +349,6 @@ units = [
         {
           'file' : 'systemd-coredumpd.service.in',
           'conditions' : ['ENABLE_COREDUMP'],
-          'symlinks' : ['sysinit.target.wants/'],
         },
         {
           'file' : 'systemd-creds.socket',

units/systemd-coredumpd.service.in

@@ -57,3 +57,6 @@ SystemCallErrorNumber=EPERM
 SystemCallFilter=@system-service @mount
 TimeoutStopSec=6min
 {{SERVICE_WATCHDOG}}
+
+[Install]
+WantedBy=sysinit.target