BTL: add flags for signaled BTL operations

As per discussion in the June 2013 developer meeting these
flags will be used by the PML in the future to request
asynchronous progress on an operation. The naming was chosen
to reflect that a BTL supports this mode (MCA_BTL_FLAG_SIGNALED)
and that a descriptor should "signal" the remote side to wake
up and progress the message (MCA_BTL_DES_FLAG_SIGNAL).

Future commits will update OB1 to take advantage of this
feature when performing the RDMA get or RDMA rendezvous
protocols.