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.

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