sh(1): make it the default shell for the root user

In the recent history sh(1) has gain the missing features for it to
become a usable interractive shell:

• command completion
• persistent history support
• improvements on the default bindings in emacs mode
• improvements in the vi mode (repect $EDITOR)
• print a newline when exiting via ^D
• default prompt and improvements on how PS1 can be configured
• and more.

This changes also simplifies making tiny freebsd images with only sh(1)
as a shell