btl/vader: improved single-copy support

This commit makes the folowing changes:

 - Add support for the knem single-copy mechanism. Initially vader will only
   support the synchronous copy mode. Asynchronous copy support may be added
   int the future.

 - Improve Linux cross memory attach (CMA) when using restrictive ptrace
   settings. This will allow Open MPI to use CMA without modifying the system
   settings to support ptrace attach (see /etc/sysctl.d/10-ptrace.conf).

 - Allow runtime selection of the single copy mechanism. The default behavior
   is to use the best available. The priority list of single-copy mehanisms is
   as follows: xpmem, cma, and knem.

 - Allow disabling support for kernel-assisted single copy.

 - Some tuning and bug fixes.
btl/vader: report an error when a segment cannot be attached
btl/vader: silence warning

correctly check HAVE_SYS_PRCTL_H
Add new PML yalla, which uses mxm directly to reduce overhead.

http://starwars.wikia.com/wiki/Ubed_Yalla
One more fix for libltdl configuration.
Remove unneeded "-I" statements from conftest compilation command.
btl/vader: fix compilation on OS X
Merge pull request #246 from yosefe/topic/pml-yalla

[RFC] Add new PML yalla, which uses mxm directly to reduce overhead.
yalla: address comments on #246 by @jsquires
pmix: correctly handle NULL OPAL_BYTE_OBJECT object
vader: vader_add_procs report the error if init_vader_endpoint fails
Revert "Provide a mechanism by which an upstream project can rename
the OPAL and ORTE libraries. This is required by projects such as ORCM
that have their own ORTE and OPAL libraries in order to avoid library
confusion. By renaming their version of the libraries, the OMPI
applications can correctly dynamically load the correct one for their
build."

This reverts commit 63f619f8719fb853d76130d667f228b0a523bd60.
configury: new OPAL_SET_LIB_PREFIX/ORTE_SET_LIB_PREFIX macros

These two macros set the prefix for the OPAL and ORTE libraries,
respectively.  Specifically, the OPAL library will be named
libPREFIXopen-pal.la and the ORTE library will be named
libPREFIXopen-rte.la.

These macros must be called, even if the prefix argument is empty.

The intent is that Open MPI will call these macros with an empty
prefix, but other projects (such as ORCM) will call these macros with
a non-empty prefix.  For example, ORCM libraries can be named
liborcm-open-pal.la and liborcm-open-rte.la.

This scheme is necessary to allow running Open MPI applications under
systems that use their own versions of ORTE and OPAL.  For example,
when running MPI applications under ORTE, if the ORTE and OPAL
libraries between OMPI and ORCM are not identical (which, because they
are released at different times, are likely to be different), we need
to ensure that the OMPI applications link against their ORTE and OPAL
libraries, but the ORCM executables link against their ORTE and OPAL
libraries.
vader: Remove stale comment