In Eshell, allow "-n" to suppress the trailing newline for "plain" echo

In Eshell, allow "-n" to suppress the trailing newline for "plain" echo

* doc/misc/eshell.texi (Built-in commands): Expand on the
documentation of echo (bug#27361).
* lisp/eshell/em-basic.el (eshell-echo): Respect OUTPUT-NEWLINE even
when 'eshell-plain-echo-behavior' is non-nil.
(eshell/echo): Add "-N" option and recommend its use over "-n" in
Lisp-friendly echo.
(eshell/printnl): Simplify; 'eshell-stringify' is equivalent to
calling 'eshell-echo' here.