I decided to proceed with installing usrmerge, so I can use the
nfs-common package in -testing. Without the usr merge, mount.nfs isn't
visible in /sbin/, so the automounter won't work right.

It went very smoothly in my testing virtual machine, so next was my
desktop, which uses encrypted LVM.

convert-usrmerge threw repeated errors about files in /lib/udev/hwdb.d/
and /lib/udev/rules.d/, which were different from the files under
/usr/lib/udev, and generally older. Eventually I just moved the two
subdirectories of /lib/udev out of the way and convert-usrmerge
completed without further complaint.

On reboot, everything appeared to start correctly, until lightdm
started, when keyboard and mouse input were ignored. Network was up, so
I logged in remotely and rebooted, forcing startup in single-user.

With /lib/udev as my exploration point from here, I decided to reinstall
eudev:

1) service start dbus
2) service start network-manager # get WLAN working
3) apt reinstall eudev

update-initramfs threw errors like:

E: /usr/share/initramfs-tools/hooks/lvm2 failed with return 1.

So I reinstalled lvm2.

Then update-initramfs complained the same but about dmsetup.

I reinstalled dmsetup.

update-initramfs completed without errors.

Rebooted.

All seems to be working now, including automounter with the -testing
branch of nfs-common.

Another day, I'll do my file server/DVR box, which does not use LVM or
encryption, so should hopefully be less trouble.

-Michael