configitems: register the 'worker.numcpus' config

mercurial/configitems.py

@@ -164,3 +164,6 b" coreconfigitem('worker', 'backgroundclos"
 coreconfigitem('worker', 'backgroundclosethreadcount',
     default=4,
 )
+coreconfigitem('worker', 'numcpus',
+    default=None,
+)