util: split out memcmp()/memset() related calls into memory-util.[ch]

Just some source rearranging.

src/basic/memory-util.c

@@ -0,0 +1,39 @@
+#include <unistd.h>
+
+#include "memory-util.h"
+
+size_t page_size(void) {
+        static thread_local size_t pgsz = 0;
+        long r;
+
+        if (_likely_(pgsz > 0))
+                return pgsz;
+
+        r = sysconf(_SC_PAGESIZE);
+        assert(r > 0);
+
+        pgsz = (size_t) r;
+        return pgsz;
+}
+
+bool memeqzero(const void *data, size_t length) {
+        /* Does the buffer consist entirely of NULs?
+         * Copied from https://github.com/systemd/casync/, copied in turn from
+         * https://github.com/rustyrussell/ccan/blob/master/ccan/mem/mem.c#L92,
+         * which is licensed CC-0.
+         */
+
+        const uint8_t *p = data;
+        size_t i;
+
+        /* Check first 16 bytes manually */
+        for (i = 0; i < 16; i++, length--) {
+                if (length == 0)
+                        return true;
+                if (p[i])
+                        return false;
+        }
+
+        /* Now we know first 16 bytes are NUL, memcmp with self.  */
+        return memcmp(data, p + i, length) == 0;
+}