Cache the isc_os_ncpu() result

It was discovered that on some platforms (f.e. Alpine Linux with MUSL)
the result of isc_os_ncpus() call differ when called before and after we
drop privileges.  This commit changes the isc_os_ncpus() call to cache
the result from the first call and thus always return the same value
during the runtime of the named.  The first call to isc_os_ncpus() is
made as soon as possible on the library initalization.

lib/isc/lib.c

@@ -14,6 +14,7 @@
 #include <isc/bind9.h>
 #include <isc/lib.h>
 #include <isc/mem.h>
+#include <isc/os.h>
 #include <isc/tls.h>
 #include <isc/util.h>
 
@@ -45,6 +46,7 @@ isc__initialize(void) {
 	isc__mem_initialize();
 	isc__tls_initialize();
 	isc__trampoline_initialize();
+	(void)isc_os_ncpus();
 }
 
 void