Log Message:
Add execshell: Proof of concept execline REPL

This is a proof-of-concept interactive REPL for Laurent Bercot's
execline language. execshell provides an interactive interface with
readline/Emacs-alike line editing to the command lexer used by the
execlineb script launcher. For example:

$ ./execshell
> foreground { echo foo } echo bar
foo
bar
> pipeline { echo baz } sed -e "s/a/u/" -e "s/$/z/"
buzz
(Note that the prompt string here is > .)