Fixes #3652: Move r28716 to v1.7 (if.c fix to check all addresses)

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r28716 [[BR]]
If we enable ipv6, we resolve a hosts addresses and check them all against our local interfaces to determine if the given host is us. However, if we don't enable ipv6, we only checked the first address returned. This can cause us to incorrectly identify a hostname as "not us".

Make -disable-ipv6 behave the same as --enable-ipv6 by checking all the returned addresses.
Fixes #3653: Move r28717, r28718 to v1.7 branch

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r28717 [[BR]]
If the default hostfile is empty, we need to pickup all the known nodes, not just the head node.

cmr:v1.7.3:reviewer=jsquyres
cmr:v1.6.6:reviewer=jsquyres


r28718 [[BR]]
Even if we don't find the default hostfile, and nothing else was provided, then use all the known nodes.

cmr:v1.7.3:#3653:reviewer=jsquyres
cmr:v1.6.6:#3654:reviewer=jsquyres
Fixes #3659: Move r28731 to v1.7.3

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r28731 [[BR]]
On some systems, pthread_kill is actually in the "signals.h" header, so include it
Fixes #3598: CMR: Move ltdl wrapper functionality to 1.7

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r28041 [[BR]]
Add wrapper functions to LTDL functions so other parts of the library can access the LTDL functionality.
Reviewed by jsquyres. 

r28042 [[BR]]
Need to include libltdl's includes so that the lt wrappers can compile
Fixes #3647: Move coll/hcoll component to v1.7 branch

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r28694 [[BR]]
Per the details and schedule outlined in the attached RFC, Mellanox Technologies would like to CMR the new 'coll/hcoll' component. This component enables Mellanox Technologies' latest HPC middleware offering - 'Hcoll'. 'Hcoll' is a high-performance, standalone collectives library with support for truly asynchronous, non-blocking, hierarchical collectives via hardware offload on supporting Mellanox HCAs (ConnectX-3 and above.) To build the component, libhcoll must first be installed on your system, then you must configure OMPI with the configure flag: '--with-hcoll=/path/to/libhcoll'. Subsequent to installing, you may select the 'coll/hcoll' component at runtime as you would any other coll component, e.g. '-mca coll hcoll,tuned,libnbc'. This has been reviewed by Josh Ladd and should be added to cmr:v1.7:reviewer=jladd

r28695 [[BR]]
Adding the ompi_check_libhcoll.m4 file

r28698 [[BR]]
Deleting garbage I accidentally committed. Thanks, Nathan\!