Fix missing and/or dangerous steps in chapter 4, simple server setup

Fixed the following issues:
- Added missing step to create /opt/git belonging to git user
- Added missing steps to set permissions on .ssh and .ssh/authorized_keys so OpenSSH doesn't ignore them by default
- Added missing step to add git-shell to /etc/shells file
- Removed dangerous advice to edit /etc/passwd to change the shell. There is a utility for this (chsh).  The passwd file should never be edited directly (at the very least, one should use vipw.  But for changing the shell only, chsh is the correct and FAR less dangerous command).
- Added specific mention of username and folders to some of the git instructions instead of "that user" to make instructions less ambiguous.