Use ErrnoMaintainer in setlocale() to protect errno.

* this avoids spurious errno changes leaking into application code,
  which could become confused - i.e. 'rm' on a gcc4 build would always
  prompt for confirmation

I spend a couple of hours hunting down the behavioural difference
between gcc2- and gcc4-builds and it turns out that the reason for that
is that gcc4's libstdc++-code initializes its own locale data via the
POSIX calls, which trigger (correct) errno value changes, which were the
ones leaking into application code.

src/system/libroot/posix/locale/setlocale.cpp

@@ -10,6 +10,8 @@
 #include <stdlib.h>
 #include <string.h>
 
+#include <ErrnoMaintainer.h>
+
 #include "LocaleBackend.h"
 
 
@@ -67,6 +69,8 @@ GetLocalesFromEnvironment(int category, const char** locales)
 extern "C" char*
 setlocale(int category, const char* locale)
 {
+	BPrivate::ErrnoMaintainer errnoMaintainer;
+
 	if (category < 0 || category > LC_LAST)
 		return NULL;