scheduler: preliminary support for generic coprocessors

A "generic" coprocessor is one that's reported by the client,
but's not of a type that the scheduler knows about (NVIDIA, AMD, Intel).

With this commit the following works:
- On the client, define a <coproc> in your cc_config.xml
  with a custom name, say 'miner_asic'.
- define a plan class such as
  <plan_class>
    <name>foobar</name>
    <gpu_type>miner_asic</gpu_type>
    <cpu_frac>0.5</cpu_frac>
  <plan_class>
- App versions of this plan class will be sent only to hosts
  that report a coproc of type "miner_asic".
  The <app_version>s in the scheduler reply will include
  a <coproc> element with the given name and count=1.
  This will cause the client (at least the current client)
  to run only one of these jobs at a time,
  and to schedule the CPU appropriately.

Note: there's a lot missing from this;
- app version FLOPS will be those of a CPU app;
- jobs will be sent only if CPU work is requested
... and many other things.
Fixing these issues requires a significant re-architecture of the scheduler,
in particular getting rid of the PROC_TYPE_* constants
and the associated arrays,
which hard-wire the 3 fixed GPU types.