keventd: add class/<subsystem>/<name> and driver/<name> conditions

dev/<X> only fires when a device gets a /dev node, which leaves a lot of
embedded-relevant hardware uncoverable: DSA switch cores, IIO sensors,
LEDs, backlight, PHYs, regulators -- all live purely under sysfs.

Two new condition namespaces:

  class/<subsystem>/<sysname>  asserted on every sysfs class device add
                                (e.g. <class/leds/blue>)
  driver/<name>                 asserted while the driver is bound to at
                                least one device (e.g. <driver/mt7530>)

A driver can bind to several devices, so driver/ conditions are
refcounted: asserted on first bind, cleared when the last device is
unbound.

dev_cond() is generalized into a static cond_emit(prefix, rel, set) so
class_cond() and driver_cond() share the same mkpath + symlink/erase
code.  The name avoids colliding with src/cond.h's public cond_path()
helper (unrelated function that returns a condition's filesystem path).

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

keventd/keventd.h

@@ -149,6 +149,9 @@ int             devnode_add      (struct uevent *ev);
 int             devnode_del      (struct uevent *ev);
 int             netdev_add       (struct uevent *ev);
 
+void            class_cond       (const struct uevent *ev, int set);
+void            driver_cond      (const struct uevent *ev, int set);
+
 int             symlink_add      (struct uevent *ev);
 int             symlink_del      (struct uevent *ev);
 void            symlink_write_db (const char *devpath, FILE *fp);