Update repository hosting documentation for all the issues users have hit

Summary:
Ref T4151. Addresses these issues:

• Mentions diffusion.ssh-user.
• Mentions /etc/shadow and !!.
• Mentions /etc/passwd and shell.
• Mentions sshd -d -d -d.
• Mentions Defaults requiretty.
• Adds AllowUsers to default configuration.
• Mentions sudo -E ... as a troubleshooting step.
• Mentions multiple VCS binaries.
• Fixes sshd paths to be absolute.
• Fixes example path in sshd_config template.
• Mentions GIT_CURL_VERBOSE.
• Walks users through cloning.
• Adds documentation for custom hooks.
• Mentions that only daemon-user interacts with repositories.
• Added general troubleshooting guide.

I didn't fix these:

• Weird one-time issue with sudoers.d/. We tell you to edit /etc/sudoers directly anyway.
• Insane #includedir magic, as above.
• Confusion around vcs-user for HTTP, since I think this is fairly clear.
• Confusion around parent directory permissions -- not sure about this one, sshd normally runs as root?

I added an ssh-shell as a safer alternative to /bin/sh. I need to test this a bit more.

Test Plan:

• Read documentation.
• Will test ssh-shell.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: bluehawk, mbishopim3, epriestley

Maniphest Tasks: T4151

Differential Revision: https://secure.phabricator.com/D8586