Fix for issue #160 (restored from last year's PR #161)

[ericdand]

I wrote this change originally to resolve issue #160, but I wrote the change into my master branch. I then used that branch to do some other development, and accidentally wiped out this change. This PR is restored from that old change.

See issue #160 and PR #161 for more background.

[google-oss-prow]

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[ericdand]

Note that in this change, since it was such a major change to the src/nss/nss_cache_oslogin.c file, also adjusts the line spacing from two spaces to four, following the Google style guide. I recommend using "ignore whitespace" while reviewing the change, to better see what has changed.

[ericdand]

This PR has now been rebased to include Ana's new "binary cache" from earlier this year. It additionally includes some changes from my recent pull requests, PR #184 and PR #185, which should probably get merged (and then folded into this PR again) first.