capture: add pid-root frame type

While I'm not thrilled to add new frame types for every sort of thing, I
think having this will be relatively useful so we can improve decoding
operations.

This adds SysprofCapturePidRoot which lets us specify a root directory
on the host system for which is the real root (/) of the PID. This can
be useful when reconstructing overlays for containers and you need to
direct access to alternate roots.

The layer gives us some ability to try to deal with overlayfs, albeit at
a very rudimentary level. In most cases I anticipate we just deal with
the main root and ignore overlays until necessary.

src/libsysprof-capture/sysprof-capture-writer.c

@@ -827,6 +827,39 @@ sysprof_capture_writer_add_fork (SysprofCaptureWriter *self,
   return true;
 }
 
+bool
+sysprof_capture_writer_add_pid_root (SysprofCaptureWriter *self,
+                                     int64_t               time,
+                                     int                   cpu,
+                                     int32_t               pid,
+                                     const char           *path,
+                                     uint32_t              layer)
+{
+  SysprofCapturePidRoot *ev;
+  size_t strl = strlen (path);
+  size_t len = sizeof *ev + strl + 1;
+
+  assert (self != NULL);
+
+  ev = (SysprofCapturePidRoot *)sysprof_capture_writer_allocate (self, &len);
+  if (!ev)
+    return false;
+
+  sysprof_capture_writer_frame_init (&ev->frame,
+                                     len,
+                                     cpu,
+                                     pid,
+                                     time,
+                                     SYSPROF_CAPTURE_FRAME_PID_ROOT);
+  ev->layer = layer;
+  memcpy (ev->path, path, strl);
+  ev->path[strl] = 0;
+
+  self->stat.frame_count[SYSPROF_CAPTURE_FRAME_PID_ROOT]++;
+
+  return true;
+}
+
 bool
 sysprof_capture_writer_add_exit (SysprofCaptureWriter *self,
                                  int64_t               time,