MPICH-specific initialization of hcoll.
In some cases, we cannot let hcoll use whatever transport it needs. For example, ch3:sock assumes that while blocking the next event will come over the socket channel. If HCOLL decides to use a different transport (such as mxm) and the next event comes on that transport, this can result in a deadlock. In this patch, we let the channel specify what transports it can accept. Signed-off-by: Devendar Bureddy <email@example.com>