core,systemctl: add bus API to retrieve processes of a unit

This adds a new GetProcesses() bus call to the Unit object which returns an
array consisting of all PIDs, their process names, as well as their full cgroup
paths. This is then used by "systemctl status" to show the per-unit process
tree.

This has the benefit that the client-side no longer needs to access the
cgroupfs directly to show the process tree of a unit. Instead, it now uses this
new API, which means it also works if -H or -M are used correctly, as the
information from the specific host is used, and not the one from the local
system.

Fixes: #2945

src/basic/process-util.c

@@ -731,6 +731,18 @@ void valgrind_summary_hack(void) {
 #endif
 }
 
+int pid_compare_func(const void *a, const void *b) {
+        const pid_t *p = a, *q = b;
+
+        /* Suitable for usage in qsort() */
+
+        if (*p < *q)
+                return -1;
+        if (*p > *q)
+                return 1;
+        return 0;
+}
+
 static const char *const ioprio_class_table[] = {
         [IOPRIO_CLASS_NONE] = "none",
         [IOPRIO_CLASS_RT] = "realtime",

src/basic/process-util.h

@@ -101,3 +101,5 @@ int sched_policy_from_string(const char *s);
 #define PID_TO_PTR(p) ((void*) ((uintptr_t) p))
 
 void valgrind_summary_hack(void);
+
+int pid_compare_func(const void *a, const void *b);