.gitignore: test directory autogenerated files

I think these files were introduced in [8a9a743f].

No reviewer.
VCs now use eager threshold from environment var.

Previously only the Nemesis shared memory eager threshold could be
changed via an environment variable.  Now the default (if not overridden
by the netmod) eager/rendezvous threshold can also be set via an
environment variable.

Reviewed-by: goodell@
add `mpiu_uthash.h`

With a couple of MPICH-specific tweaks.

The `mx` netmod uses a non-namespaced "uthash.h" file based on an older
version of the package.  This is probably OK for now, but we run the
risk of symbol conflict and the update is nontrivial since the API
changed between the two versions.

No reviewer.
Add info hint support to communicators.

Communicators now accept info hints both at creation time
(`MPI_Comm_dup_with_info`) as well as at any later point in their
life cycle (`MPI_Comm_set_info`).

Hints are stored in a hash table, and may be added by defining a
function of type

  int fun(MPID_Comm *, MPID_Info *, void *)

which applies the hint to the communicator, and then registered with
MPICH via the function `MPIR_Comm_register_hint_fn`. The third
(optional) argument can be used to pass extra data to the handler, such
as local context from when it was registered with the runtime.

Reviewed-by: goodell
Add per-communicator eager threshold support.

Message transfers now respect the communicator-specific threshold.  This
change has not been carefully checked for impact on our shared-memory
ping-pong latency.

Reviewed-by: goodell