Take unix domain socket max length into account
Take unix domain socket max length into account

Summary: lawrencelomax helpfully pointed out that there's a legacy limit for the path length of unix domain sockets. Checking here and falling back to `/tmp` in case we're going over. This could have caused some gnarly support issues, so I'm glad we caught this before it went live.

Reviewed By: aigoncharov

Differential Revision: D35257794

fbshipit-source-id: 68a7b62d6d6863efa4b3ce84d7735b1c1a45a174

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