cmdmon: make open commands configurable

Replace the hardcoded list of open commands (accessible over UDP),
with a list that can be configured with a new "opencommands" directive.
The default matches the original list. All read-only commands except
accheck and cmdaccheck can be enabled. The naming follows the chronyc
naming. Enable the N_SOURCES request only when needed.

This makes it possible to have a full monitoring access without access
to the Unix domain socket. It also allows restricting the monitoring
access to a smaller number of commands if some commands from the default
list are not needed.

Mention in the man page that the protocol of the non-default commands is
not consider stable and the information they provide may have security
implications.

doc/chrony.conf.adoc

@@ -2143,6 +2143,27 @@ An example of the use of the directive is:
 cmdratelimit interval 2
 ----
 
+[[opencommands]]*opencommands* [_command_]...::
+This directive specifies a list of monitoring commands to be enabled for the
+hosts allowed by the *cmdallow* directive. The following commands can be
+specified (the naming follows *chronyc*):
++
+*activity*+*+, *authdata*, *clients*, *manual*+*+, *ntpdata*, *rtcdata*+*+,
+*selectdata*, *serverstats*+*+, *smoothing*+*+, *sourcename*+*+, *sources*+*+,
+*sourcestats*, *tracking*+*+.
++
+The commands marked with +*+ are enabled by default. The protocol of these
+commands is considered stable and can be expected to work between different
+versions of *chronyc* and *chronyd*. The protocol of the other commands is not
+considered stable and different versions of *chronyc* and *chronyd* may not
+interoperate. When that happens, *chronyc* will print an '`Invalid command`' or
+'`Bad reply from daemon`' error.
++
+Note that some of the reported data can be potentially useful to attackers,
+enabling them to better observe and predict the internal state of *chronyd*.
+It is recommended to enable only commands that are actually needed for
+monitoring and limit the access to the hosts that need it.
+
 === Real-time clock (RTC)
 
 [[hwclockfile]]*hwclockfile* _file_::