valid service/env/whence tuples are specified in the client_services
table.  Only services that exist in that table can be configured -- so
if rc.ifconfig is only configurable at boot, every time, the user can
only add hooks (or control the service) for rc.ifconfig at boot, every
time (so not in the load env, nor only once).

Users can either specify a script that gets turned into a per-experiment
blob, OR they can specify a static blob that they already created in the
blobs table via mkblob.  They can't do both though.

tb-set-node-service "rc.foo" \
    -node (""|$node) -env (boot|load) -whence (every|first) \
    -script "/path/to/script" -scriptblob "<blobid>" \
    -enable (0|1) -enablehooks (0|1) -fatal (0|1)

All options are "optional".  Even if you disable a service, its hooks
are enabled by default.  Services can now be fatal.

tb-add-node-service-hook "rc.foo" \
    -node (""|$node) -env (boot|load) -whence (every|first) \
    -script "/path/to/script" -scriptblob "<blobid>" \
    -op (boot|shutdown|reconfig|reset) -point (pre|post) \
    -argv "" -fatal (0|1)