do not use a global variable to define the number of available threads

* python/spot/impl.i: Make parallel_policy implicitly contractible.
* spot/graph/graph.hh (sort_edges_srcfirst_): Pass a parallel_policy
explicitly.
* spot/twa/twagraph.hh, spot/twa/twagraph.cc (merge_states): Likewise.
* spot/misc/common.cc: Remove file.
* spot/misc/common.hh (set_nthreads, get_nthreads): Remove, and
replace with...
(parallel_policy): ... this.
* spot/misc/Makefile.am, tests/python/mergedge.py, NEWS: Adjust.

NEWS

@@ -38,9 +38,6 @@ New in spot 2.10.6.dev  (not yet released)
 
   Library:
 
-  - A global variable, together with its setters and getters to define the
-    maximal number of threads is added to common.hh/common.cc
-
   - The new function suffix_operator_normal_form() implements
     transformation of formulas to Suffix Operator Normal Form,
     described in [cimatti.06.fmcad].
@@ -117,6 +114,11 @@ New in spot 2.10.6.dev  (not yet released)
     to obtain a simple model checker (that returns true or false,
     without counterexample).
 
+  - spot::parallel_policy is an object that can be passed to some
+    algorithm to specify how many threads can be used if Spot has been
+    compiled with --enable-pthread.  Currently, only
+    twa_graph::merge_states() support it.
+
  Python bindings:
 
   - The to_str() method of automata can now export a parity game into