login: Use pread64 in utmp implementation

This reduces the possible error scenarios considerably because
no longer can file seek fail, leaving the file descriptor in an
inconsistent state and out of sync with the cache.

As a result, it is possible to avoid setting file_offset to -1
to make an error persistent.  Instead, subsequent calls will retry
the operation and report any errors returned by the kernel.

This change also avoids reading the file from the start if pututline
is called multiple times, to work around lock acquisition failures
due to timeouts.

Change-Id: If21ea0c162c38830a89331ea93cddec14c0974de
(cherry picked from commit d4625a19)