sys: add OpenBSD support

Add OpenBSD support, including pledge(2) support by implementing
SYS_EnableSystemCallFilter().

This commit depends on the addition of AdjustFreq() privops and the
addtion of invoking SYS_EnableSystemCallFilter() from PRV_StartHelper().

Only system call filter levels on/off' are supported. Setting level
to 0 disables the filter and setting it to 1 enables it.

Update the documentation to reflect that OpenBSD supports:
- the SCHED_FIFO real-time scheduler (option -P)
- locking chronyd into memory (option -m)
- reload sample history of servers and ref clocks (option -r)
- forking into two process when run as non-root user (option -u)
- maxdrift/maxslewrate of 100000.

sys_openbsd.h

@@ -0,0 +1,41 @@
+/*
+  chronyd/chronyc - Programs for keeping computer clocks accurate.
+
+ **********************************************************************
+ * Copyright (C) Shaun Ren  2021
+ * Copyright (C) Thomas Kupper  2026
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of version 2 of the GNU General Public License as
+ * published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+ *
+ **********************************************************************
+
+  =======================================================================
+
+  Header file for OpenBSD driver
+  */
+
+#ifndef GOT_SYS_OPENBSD_H
+#define GOT_SYS_OPENBSD_H
+
+#include "sys.h"
+
+void SYS_OpenBSD_Initialise(void);
+
+void SYS_OpenBSD_Finalise(void);
+
+void SYS_OpenBSD_DropRoot(uid_t uid, gid_t gid, SYS_ProcessContext context, int clock_control);
+
+void SYS_OpenBSD_EnableSystemCallFilter(int level, SYS_ProcessContext context);
+
+#endif