Fix mxb_strerror

The function should not be an inline function with a static variable. This
appears to cause problems on at least Debian Wheezy and is likely to cause
odd behavior on other platforms.

Also renamed the file to <maxbase/string.h> to better mirror how the
<string.h> file behaves.

maxutils/maxbase/include/maxbase/string.h

@@ -14,16 +14,7 @@
 
 #include <maxbase/cdefs.h>
 
-#include <string.h>
-
-inline const char* mxb_strerror(int error)
-{
-    // Enough for all errors
-    static thread_local char errbuf[512];
-#ifdef HAVE_GLIBC
-    return strerror_r(error, errbuf, sizeof(errbuf));
-#else
-    strerror_r(error, errbuf, sizeof(errbuf));
-    return errbuf;
-#endif
-}
+/**
+ * Thread-safe strerror
+ */
+const char* mxb_strerror(int error);