handle situation where there is no environment variables / HOME at all

[maage]

At least current openssh 7.8p1 does use ssh-pkcs11-helper and it does not have any envs defined. To be able to read config we need to get home so get it via getpwuid. I did not test failure modes, but it is possible to have user without entry in the passwd.
Without this patch ssh-add -s fails and ssh-pkcs11-helper either dies or dumps core. As ssh-agent does not reap or restart helpers it is kind of bad.

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[ThomasHabets]

I'm still waiting for CLA