core: add a hwlocVersion attribute to the topology

When loading from XML, don't overwrite it.

This is useful for debugging warnings/problems when
passing XMLs between processes such as Slurm, OMPI, ...
core: add hwloc_progname() to retrieve the process name

Will help figuring out where errors and/or buggy XML/topologies come from.
core: Add a ProcessName attribute to topologies
core: display versions and process names in out-of-order XML import errors

When getting out-of-order XML import errors because mixing hwloc versions
in complex environment such as MPI and Slurm, display the current and
original hwloc version and process name.
VERSION: replace gitclone with 2.0.0a1-git

Make config/hwloc_get_version.sh replace some ${foo} as well as ^foo.
Don't replace all occurences of foo to avoid breaking something else by mistake.