As suggested by Vladislav Vaintroub, let us remove misleading
and malformatted startup messages.

Even if the global variable srv_use_atomic_writes were set, we would
still invoke my_test_if_atomic_write() to check if writes are atomic
with a particular page size.

When using the default innodb_page_size=16k, page writes should be
atomic on NTFS when using ROW_FORMAT=COMPRESSED and KEY_BLOCK_SIZE<=4.

Disabling srv_use_atomic_writes when innodb_file_per_table=OFF does
not make sense, because that is a dynamic parameter.

We also correct the documentation string of innodb_use_atomic_writes
and remove the duplicate variable innobase_use_atomic_writes.