keventd: record the libblkid build requirement

keventd is the only thing in the tree that links libblkid, so a tree
that used to build now stops in configure with no hint of which package
to install.  Say so where people look for dependencies, and give CI the
package it now needs.

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

doc/keventd.md

@@ -6,8 +6,11 @@ with Finit.  It replaces the need for external device managers like
 mdev, mdevd, or udevd on systems where a lighter-weight solution is
 preferred, particularly on embedded systems.
 
-It is enabled by default since Finit v5.  To disable it and use an
-external device manager instead:
+It is enabled by default since Finit v5, and needs libblkid from
+util-linux (the `libblkid-dev` package, or whatever your distribution
+calls it) to read the filesystem UUID and label behind the
+`/dev/disk/by-uuid/` and `/dev/disk/by-label/` symlinks.  To disable it
+and use an external device manager instead:
 
     ./configure --without-keventd
 
@@ -103,6 +106,10 @@ among others:
 They also set up a few fixed names: `/dev/rtc`, `/dev/cdrom`, and
 `/dev/virtio-ports/<name>`.
 
+The `by-uuid` and `by-label` links are why keventd needs libblkid.  The
+rules read the filesystem metadata off the device with
+`IMPORT{builtin}="blkid"`, and libblkid is what that builtin calls.
+
 Built-in and rule-provided symlinks alike are tracked internally and
 removed when the corresponding device is unplugged.
 
@@ -252,8 +259,6 @@ Usage
       -G        Disable netlink rebroadcast entirely
       -h        Show help text
       -n        Run in foreground (no daemon)
-      -p        Passive mode: power supply events only
-      -r DIR    Extra rules directory
       -v        Show version
 
 In normal operation, Finit starts keventd automatically via its system