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.

test/system/199-scfilter

@@ -8,7 +8,21 @@ check_chronyd_features SCFILTER || test_skip "SCFILTER support disabled"
 
 test_start "system call filter in destructive tests"
 
-for level in 1 2 -1 -2; do
+os_name="$(uname -s)"
+case $os_name in
+	OpenBSD)
+		supported_levels=1
+		;;
+	Linux)
+		supported_levels="1 2 -1 -2"
+		;;
+	*)
+		test_message 1 1 "unsupported OS $os_name"
+		test_fail
+		;;
+esac
+
+for level in $supported_levels; do
 	test_message 1 1 "level $level:"
 	for test in 1[0-8][0-9]-*[^_]; do
 		test_message 2 0 "$test"