selinux: add bus service access control

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This patch adds the ability to look at the calling process that is trying to
do dbus calls into systemd, then it checks with the SELinux policy to see if
the calling process is allowed to do the activity.

The basic idea is we want to allow NetworkManager_t to be able to start and
stop ntpd.service, but not necessarly mysqld.service.

Similarly we want to allow a root admin webadm_t that can only manage the
apache environment.  systemctl enable httpd.service, systemctl disable
iptables.service bad.

To make this code cleaner, we really need to refactor the dbus-manager.c code.
 This has just become a huge if-then-else blob, which makes doing the correct
check difficult.

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src/core/bus-errors.h

@@ -43,6 +43,7 @@
 #define BUS_ERROR_TRANSACTION_ORDER_IS_CYCLIC "org.freedesktop.systemd1.TransactionOrderIsCyclic"
 #define BUS_ERROR_SHUTTING_DOWN "org.freedesktop.systemd1.ShuttingDown"
 #define BUS_ERROR_NO_SUCH_PROCESS "org.freedesktop.systemd1.NoSuchProcess"
+#define BUS_ERROR_ACCESS_DENIED "org.freedesktop.systemd1.AccessDenied"
 
 static inline const char *bus_error(const DBusError *e, int r) {
         if (e && e->message)