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). 
- Rewrote the advice to edit the /etc/passwd file directly so it says to never do that and to use the vipw utility.
- Added specific mention of username and folders to some of the git instructions instead of "that user" to make instructions less ambiguous.