Setting LB_ARGS="..." was causing non-defined options to be empty
instead of parse the default value

Do a true reset in shmem_cpuinfo by acquiring all owned CPUs and
lending the rest. The previous logic was wrong, a simple AcquireCpuMask
is not enoutgh.

In polling mode, DLB must always enable the CPU and let the runtime decide
whether there will be CPU sharing by calling checkCPUAvailability
Also, fix needed in shmem_cpuinfo__is_cpu_available to support the current
mechanism in AcquireCpu

HWLOC_LIBDIR needs to be exported without link flags. For that reason,
a bit of refactoring has been needed in ax_hwloc

Several flags are needed, and CFLAGS cannot be shared since there
are some incompatible flags

Now all policy functions must return an integer. If the function is not defined,
a new error value DLB_ERR_NOPOL is returned.

A list of CPUs is constructed every time the process mask
changes. Only if PRIO_AFFINITY_FULL, the scheduled CPUs are
dependant on the CPU state.

It was unclear deciding which condition did what.
Now each one will return a cpuset of parents that either
cover o are inside a base cpuset.

We cannot request usage of an undetermined CPU based only on scheduling.
The APIs Borrow & BorrowCpus will provide the intended use case now.

We need something more generic for scheduling resources.
For now, simply construct a list of candidates based on Owned + Rest

$srcdir/.git can be a file in case of git worktrees