sysusers: emit audit events for user and group creation (#35957)

Background: Fedora/RHEL are switching to sysusers.d metadata for
creation of users and groups for system users defined by packages
(https://fedoraproject.org/wiki/Changes/RPMSuportForSystemdSysusers).
Packages carry sysusers files. During package installation, rpm calls an
program to execute on this config. This program may either be
/usr/lib/rpm/sysusers.sh which calls useradd/groupadd, or
/usr/bin/systemd-sysusers. To match the functionality provided by
useradd/groupadd from the shadow-utils project, systemd-sysusers must
emit audit events so that it provides a drop-in replacement.

systemd-sysuers will emit audit events AUDIT_ADD_USER/AUDIT_ADD_GROUP
when adding users and groups. The operation "names" are copied from
shadow-utils, so the format of the events that is generated on success
should be identical. On failure, things are more complicated. We write
the whole file at once, once, so we first generate "success" messages
for each entry, then we try to write the files, and if things fail, we
generate failure messages to all entries that we failed to write.

src/core/manager.c

@@ -3560,7 +3560,7 @@ void manager_send_unit_audit(Manager *m, Unit *u, int type, bool success) {
         if (MANAGER_IS_RELOADING(m))
                 return;
 
-        audit_fd = get_audit_fd();
+        audit_fd = get_core_audit_fd();
         if (audit_fd < 0)
                 return;
 
@@ -3575,7 +3575,7 @@ void manager_send_unit_audit(Manager *m, Unit *u, int type, bool success) {
                 if (ERRNO_IS_PRIVILEGE(errno)) {
                         /* We aren't allowed to send audit messages?  Then let's not retry again. */
                         log_debug_errno(errno, "Failed to send audit message, closing audit socket: %m");
-                        close_audit_fd();
+                        close_core_audit_fd();
                 } else
                         log_warning_errno(errno, "Failed to send audit message, ignoring: %m");
         }