Fixes for atomic writes on Windows.

Windows does atomic writes, as long as they are aligned and multiple
of sector size. this is documented in MSDN.

Fix innodb.doublewrite test to always use doublewrite buffer,
(even if atomic writes are autodetected)

mysql-test/suite/innodb/t/doublewrite.combinations

 [strict_crc32]
 --innodb-checksum-algorithm=strict_crc32
+--innodb-use-atomic-writes=0
 
 [strict_full_crc32]
 --innodb-checksum-algorithm=strict_full_crc32
+--innodb-use-atomic-writes=0

mysys/my_atomic_writes.c

@@ -15,7 +15,7 @@
 
 #include "mysys_priv.h"
 
-my_bool my_may_have_atomic_write= 0;
+my_bool my_may_have_atomic_write= IF_WIN(1,0);
 
 #ifdef __linux__
 

storage/innobase/os/os0file.cc

@@ -7669,7 +7669,15 @@ void fil_node_t::find_metadata(os_file_t file
 			&& my_test_if_atomic_write(file,
 						   space->physical_size())
 #else
+			/* On Windows, all single sector writes are atomic,
+			as per WriteFile() documentation on MSDN.
+			We also require SSD for atomic writes, eventhough
+			technically it is not necessary- the reason is that
+			on hard disks, we still want the benefit from
+			(non-atomic) neighbor page flushing in the buffer
+			pool code. */
 			&& srv_page_size == block_size
+			&& on_ssd
 #endif
 			;
 	}