add existing documentation

doc/system-services.txt

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+==== SERVICE LIST ==============================================================
+
+overlordd	root process, manages all system-level services
+launchd		launches executables
+nsd		manages the system namespace (aka where all filesystems are
+		mounted)
+lockdownd	implements security and authentication
+sessiond	manager and root process for a user session
+airportd	manages sockets and network communications.
+emdevd		host process for device drivers, one instance per system, hosts
+		*all* devices
+seatd		Manages and controls access to monitor and peripheral devices.
+fsd		host process for filesystem driver, one instance per mounted
+		filesystem
+
+
+==== TASK HIERARCHY ============================================================
+
+backboardd
++--launchd
++--nsd
++--lockdownd
++--deviced
++--airportd
++--sysfsd
++--seatd
++--sessiond
+|  +--window-manager
+|     +--file-browser
+|     +--web-browser
++--sessiond
+|  +--fbterm
+|     +--bshell
+|        +--vim
++--e2fsd
++--fatfsd
+
+
+==== SERVICE DETAILS ===========================================================
+
+## BACKBOARDD ####
+ * Root task (ID 1) created by the kernel during boot
+ * manages the startup and lifecycle of all system-level services
+ * Every task is provided with a handle to backboardd as part of the initial
+   task environment
+ * Backboardd can be used to gain access to certain system-level services, such
+   as the namespace and task launcher
+
+## LAUNCHD ####
+ * Task creation service
+ * Accepts requests from clients to run executable files
+
+## NSD ####
+ * Manages the system namespace
+ * Filesystems can be mounted in the system namespace to make them visible to
+   clients.
+
+## LOCKDOWND ####
+ * Manages security and authentication
+ * Provides security tokens that can be associated with privileges and used
+   for authorisation.
+
+## SESSIOND ####
+ * Root process for user login sessions.
+ * All child tasks receive a connection to the session controller.
+ * Holds session-wide data
+
+## DEVICED ####
+ * Hosts all device drivers.
+ * Device drivers are implemented as shared libraries that are loaded into
+   the device server at runtime.
+ * Also hosts the device filesystem (usually mounted at /dev)
+
+## SEATD ####
+ * Manages 'seats'.
+ * A seat is all of the peripherals that a user needs to interact with the
+   system. Includes the monitor, keyboard, and mouse.
+ * Allows multiple tasks to access a single seat at once. The user can use a
+   special keyboard shortcut to switch between tasks.
+
+## AIRPORTD ####
+ * Manages remote communications.
+ * All open sockets are represented internally as connections to airportd.
+ * Supports loadable modules, which provide support for different network features,
+   such as unix sockets, internet sockets, and passing messages between remote machines.
+