Fix by purging and re-installing
I could not figure out the particular problem, but I was able to recover by purging all of the packages associated with bumblebee (I used the "Complete Removal" tag in Synaptic, which I believe is equivalent to the "purge" command in apt-get). I removed all packages matching "bumblebee", "nvidia", "primus", and one called "bbswitch-dkms". I re-installed bumblebee, bumblebee-nvidia, and mesa-tools. At that point, I could run "optirun glxgears" and "primus glxgears".
I still find that other applications --- Cool VL Viewer and Imprudence --- which ran under optirun before my upgrade to Saucy now crash. Cool VL Viewer runs without optirun.
Also fix some library path problems
Cool VL Viewer and Imprudence required libraries i965.so and swrast.so, which were not on the paths that they searched.
I worked around the problem, without understanding the root of it, by entering two symbolic links:
/usr/lib64/dri -> /usr/lib/x86_64-linux-gnu/dri
/usr/lib32/dri -> /usr/lib/i386-linux-gnu/dri
I'm not sure whether to regard this as a Bumblebee problem, but the fact that Cool VL viewer worked without optirun, then failed with optirun, and the fact that the unfound libraries appear to relate to Intel graphics rather than Nvidia, suggest that the redirection of Intel calls is failing under some circumstances.