Don't override customized environments in subshells

Only set up environment for the top-level shell, and allow it to be
inherited normally.

A top level shell is usually a login shell, but can also be a
non-interactive, non-login shell in certain cases, such as when
executing an SSH remote command.
Comment a typeset
Set Mac OS X 10.6 terminal window title
Remove unused GNU Screen window number format
Rewrite terminal auto-title
Simplify auto-titling
Prefix internal functions
Unsuffix internal functions
Add configurable terminal window and tab title formats
Prefix rails aliases with 'bundle exec'
Update external syntax-highlighting
Update external history-substring-search
Update external completions