Windows: Rewrite LargeSearchTime conversions

Use LARGE_INTEGER to avoid the need for shifting and DWORD casts.
Take into account the size of time_t.

Change-Id: I056d920894b661ebb2060a5010efd9a0cd5a4a5d
Reviewed-on: http://gerrit.openafs.org/10673
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>