Mark partially purged arena chunks as non-hugepage.

Add the pages_[no]huge() functions, which toggle huge page state via
madvise(..., MADV_[NO]HUGEPAGE) calls.

The first time a page run is purged from within an arena chunk, call
pages_nohuge() to tell the kernel to make no further attempts to back
the chunk with huge pages.  Upon arena chunk deletion, restore the
associated virtual memory to its original state via pages_huge().

This resolves #243.

test/unit/pages.c

@@ -0,0 +1,27 @@
+#include "test/jemalloc_test.h"
+
+TEST_BEGIN(test_pages_huge)
+{
+	bool commit;
+	void *pages;
+
+	commit = true;
+	pages = pages_map(NULL, PAGE, &commit);
+	assert_ptr_not_null(pages, "Unexpected pages_map() error");
+
+	assert_false(pages_huge(pages, PAGE),
+	    "Unexpected pages_huge() result");
+	assert_false(pages_nohuge(pages, PAGE),
+	    "Unexpected pages_nohuge() result");
+
+	pages_unmap(pages, PAGE);
+}
+TEST_END
+
+int
+main(void)
+{
+
+	return (test(
+	    test_pages_huge));
+}