usnic: fix connectivity checker timeout

Fix mismatch between the MCA param (which expresses the timeout in
*mili*seconds) and the struct timeval timeout (which expresses the
timeout in *micro*seconds).

Reviewed by Dave Goodell

cmr=v1.8.2:reviewer=ompi-rm1.8
usnic: reduce sizeof(ompi_btl_usnic_addr_t) to 56 bytes
    
Trivial struct re-ordering to eliminate holes in the middle of the
struct (although there's still a hole at the end) and reduce the
overall size of the struct from 64 to 56 bytes.  Also change mtu from
int to uint16_t; there was no need for it to be that large.

Reviewed by Dave Goodell

cmr=v1.8.2:reviewer=ompi-rm1.8
usnic: ensure that procs agree on use_udp value
    
Add the component use_udp value into the modex.  If my component's
use_udp value doesn't agree with the use_udp value from a peer's modex
data, print a helpful message and disqualify the usnic BTL (the usnic
BTL will not be used).  This prevents accidental customer
misconfigurations.

Reviewed by Dave Goodell

cmr=v1.8.2:reviewer=ompi-rm1.8