[SSSD] [RFCv2] NSS tlog integration

[Nikolai Kondrashov]

Hi everyone,

Here is the second version of RFC for SSSD/tlog integration. This one covers
support for all necessary NSS requests: getpwnam, getpwuid and getpwent.

I don't think it handles caching well, is probably not very fast, has little
tests, and still tries to document code, apart from likely being wrong in many
parts otherwise, but I would like to show it to you, so you can correct me
earlier rather than later.

I'll proceed to the PAM part and adding more tests in the meantime.

Thank you!

Nick