usnic: add btl_usnic_connectivity_map MCA param to output link information

If the btl_usnic_connectivity_map MCA param is set to a non-NULL
value, then each MPI process will output a file named
<prefix>-<hostname>.pid<pid>.job<jobid>.mcwrank<MCW rank>.txt.  Its
contents will detail which usNIC device(s) (and therefore which
link(s)) are being used to communicate with each peer MPI process.

Here is a sample output file (named
mpi005.pid26071.job1640759297.mcwrank0.txt):

{{{
device=usnic_0,interface=eth4,ip=10.10.0.5/16,mac=24:57:20:05:20:00,mtu=9000
device=usnic_1,interface=eth5,ip=10.2.0.5/16,mac=24:57:20:05:21:00,mtu=9000
device=usnic_2,interface=eth6,ip=10.3.0.5/16,mac=24:57:20:05:50:00,mtu=9000
peer=1,hostname=mpi006,device=usnic_0@peer_ip=10.10.0.6/16@peer_mac=24:57:20:06:20:00,device=usnic_1@peer_ip=10.2.0.6/16@peer_mac=24:57:20:06:21:00,device=usnic_2@peer_ip=10.3.0.6/16@peer_mac=24:57:20:06:50:00
peer=2,hostname=mpi007,device=usnic_0@peer_ip=10.10.0.7/16@peer_mac=24:57:20:07:20:00,device=usnic_1@peer_ip=10.2.0.7/16@peer_mac=24:57:20:07:21:00,device=usnic_2@peer_ip=10.3.0.7/16@peer_mac=24:57:20:07:50:00
peer=3,hostname=mpi008,device=usnic_0@peer_ip=10.10.0.8/16@peer_mac=24:57:20:08:20:00,device=usnic_1@peer_ip=10.2.0.8/16@peer_mac=24:57:20:08:21:00,device=usnic_2@peer_ip=10.3.0.8/16@peer_mac=24:57:20:08:50:00
}}}

Reviewed by Reese Faucette

cmr=v1.8.2
mca_base_var: use ERR_NOT_FOUND when var is inactive

After discussion with Nathan, change the ERR_VALUE_OUT_OF_BOUNDS to be
ERR_NOT_FOUND, for two reasons:

1. It's consistent with other uses of ERR_NOT_FOUND in the code.
1. In this case, we could have just looked up a variable that is
   basically a "hole" -- e.g., var indexes 0, 1, 2 are valid, and 4,
   5, and 5 are valid, but 3 is invalid (e.g., 3 was de-registered --
   remember that MPI_T explicitly does not allow re-using indexes).
   So returning ERR_OUT_OF_BOUNDS seems weird -- returning
   ERR_NOT_FOUND seems a bit more natural.

cmr=v1.8.2:ticket=#4587
btl_usnic_map.c: Arrgh!  Forgot to svn add this file.

cmr=v1.8.2:ticket=#4773