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>

doc/conditions.md

@@ -177,7 +177,8 @@ Built-in Conditions
 
 Finit comes with a set of plugins for conditions:
 
- - `keventd`: provides `<dev/...>` and `<sys/pwr/...>`
+ - `keventd`: provides `<dev/...>`, `<class/...>`, `<driver/...>`, and
+   `<sys/pwr/...>`
  - `devmon` (built-in fallback for `<dev/...>` without keventd)
  - `netlink`: provides `<net/...>`
  - `pidfile`: provides `<pid/...>`
@@ -191,6 +192,11 @@ condition is cleared.  If keventd is not in use (an external device
 manager like udevd is used instead), the `devmon` built-in provides the
 same conditions by monitoring `/dev` and `/dev/dir` with inotify.
 
+keventd also asserts `class/<subsystem>/<name>` for devices that have
+no `/dev` node, e.g. LEDs and DSA switch ports, and `driver/<name>`
+while a driver is bound to at least one device.  See
+[keventd](keventd.md) for details.
+
 The `pidfile` plugin recursively watches `/run/` for PID files created
 by the monitored services, and sets a corresponding condition in the
 `pid/` namespace.
@@ -234,6 +240,8 @@ Built-in conditions:
 - `usr/foo`
 - `boot/arg`
 - `dev/node` and `dev/dir/node`
+- `class/<subsystem>/<name>`
+- `driver/<name>`
 
 > [!NOTE]
 > Here, `up` means administratively up, the interface flag `IFF_UP`.