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.

doc/chrony.conf.adoc

@@ -1449,8 +1449,8 @@ for a high quality clock using a temperature compensated crystal oscillator.
 This directive specifies the maximum assumed drift (frequency error) of the
 system clock. It limits the frequency adjustment that *chronyd* is allowed to
 use to correct the measured drift. It is an additional limit to the maximum
-adjustment that can be set by the system driver (100000 ppm on Linux, 500 ppm
-on FreeBSD, NetBSD, and macOS 10.13+, 32500 ppm on illumos).
+adjustment that can be set by the system driver (100000 ppm on Linux and
+OpenBSD, 500 ppm on FreeBSD, NetBSD, and macOS 10.13+, 32500 ppm on illumos).
 +
 By default, the maximum assumed drift is 500000 ppm, i.e. the adjustment is
 limited by the system driver rather than this directive.
@@ -1487,10 +1487,10 @@ is effective only on systems where *chronyd* is able to control the rate (i.e.
 all supported systems with the exception of macOS 12 or earlier).
 +
 For each system there is a maximum frequency offset of the clock that can be set
-by the driver. On Linux it is 100000 ppm, on FreeBSD, NetBSD and macOS 10.13+ it
-is 5000 ppm, and on illumos it is 32500 ppm. Also, due to a kernel limitation,
-setting *maxslewrate* on FreeBSD, NetBSD, macOS 10.13+ to a value between 500
-ppm and 5000 ppm will effectively set it to 500 ppm.
+by the driver. On Linux and OpenBSD it is 100000 ppm, on FreeBSD, NetBSD and
+macOS 10.13+ it is 5000 ppm, on illumos it is 32500 ppm. Also, due to a kernel
+limitation, setting *maxslewrate* on FreeBSD, NetBSD, macOS 10.13+ to a value
+between 500 ppm and 5000 ppm will effectively set it to 500 ppm.
 +
 By default, the maximum slew rate is set to 83333.333 ppm (one twelfth).
 
@@ -2988,7 +2988,7 @@ file when the <<chronyc.adoc#rekey,*rekey*>> command is issued by *chronyc*).
 [[lock_all]]*lock_all*::
 The *lock_all* directive will lock the *chronyd* process into RAM so that it
 will never be paged out. This can result in lower and more consistent latency.
-The directive is supported on Linux, FreeBSD, NetBSD, and illumos.
+The directive is supported on Linux, FreeBSD, NetBSD, OpenBSD, and illumos.
 
 [[pidfile]]*pidfile* _file_::
 Unless *chronyd* is started with the *-Q* option, it writes its process ID
@@ -3039,11 +3039,11 @@ accepted NTP-over-PTP messages. Messages from other domains are ignored.
 The default is 123, the minimum is 0, and the maximum is 255.
 
 [[sched_priority]]*sched_priority* _priority_::
-On Linux, FreeBSD, NetBSD, and illumos, the *sched_priority* directive will
-select the SCHED_FIFO real-time scheduler at the specified priority (which must
-be between 0 and 100). On macOS, this option must have either a value of 0 (the
-default) to disable the thread time constraint policy or 1 for the policy to be
-enabled.
+On Linux, FreeBSD, NetBSD, OpenBSD, and illumos, the *sched_priority* directive
+will select the SCHED_FIFO real-time scheduler at the specified priority (which
+must be between 0 and 100). On macOS, this option must have either a value of 0
+(the default) to disable the thread time constraint policy or 1 for the policy
+to be enabled.
 +
 On systems other than macOS, this directive uses the *pthread_setschedparam()*
 system call to instruct the kernel to use the SCHED_FIFO first-in, first-out
@@ -3065,9 +3065,9 @@ The *user* directive sets the name of the system user to which *chronyd* will
 switch after start in order to drop root privileges.
 +
 On Linux, *chronyd* needs to be compiled with support for the *libcap* library.
-On macOS, FreeBSD, NetBSD and illumos *chronyd* forks into two processes.
-The child process retains root privileges, but can only perform a very limited
-range of privileged system calls on behalf of the parent.
+On macOS, FreeBSD, NetBSD, OpenBSD, and illumos *chronyd* forks into two
+processes. The child process retains root privileges, but can only perform a
+very limited range of privileged system calls on behalf of the parent.
 +
 The compiled-in default value is _@DEFAULT_USER@_.