keventd: add passive mode (-p) for coexistence with hotplug plugin

This is a backwards compatible mode for users upgrading and not noticing
that keventd is now build by default.

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

doc/keventd.md

@@ -221,7 +221,7 @@ battery:
 Usage
 -----
 
-    keventd [-cdGhnv] [-g GROUP]
+    keventd [-cdGhnpv] [-g GROUP]
 
     Options:
       -c        Run coldplug at startup
@@ -230,6 +230,7 @@ Usage
       -G        Disable netlink rebroadcast entirely
       -h        Show help text
       -n        Run in foreground (no daemon)
+      -p        Passive mode: power supply events only
       -v        Show version
 
 In normal operation, Finit starts keventd automatically via its system
@@ -259,11 +260,12 @@ This means keventd can also be tested independently:
     # Run without rebroadcast (e.g., headless embedded system)
     keventd -c -G
 
-When keventd is enabled, it conflicts with external device managers.
-Only one device manager should be active at a time.  The system
-configuration uses the `conflict:` directive to enforce this:
-
-    service conflict:udevd,mdevd,mdev [...] keventd -c -- Finit device manager
+Only one device manager should be active at a time.  This is settled
+at build time: with the hotplug plugin enabled, Finit starts keventd
+in passive mode (`-p`), leaving device management to the udevd, mdevd,
+or mdev service from `system/10-hotplug.conf` and monitoring only
+power supply events.  Without the hotplug plugin, keventd runs as the
+system device manager (`keventd -c`).
 
 [libudev-zero]: https://github.com/illiliti/libudev-zero
 [mdevd]:        https://skarnet.org/software/mdevd/