Improve the behavior of concatenating parts of Eshell arguments

Improve the behavior of concatenating parts of Eshell arguments

Previously, concatenating a list to a string would first convert the
list to a string.  Now, the string is concatenated with the last
element of the list.

* lisp/eshell/esh-util.el (eshell-to-flat-string): Make obsolete.

* lisp/eshell/esh-arg.el (eshell-concat, eshell-concat-1): New
functions.
(eshell-resolve-current-argument): Use 'eshell-concat'.

* test/lisp/eshell/esh-var-tests.el (esh-var-test/interp-concat-cmd):
Add check for concatenation of multiline output of subcommands.
(esh-var-test/quoted-interp-concat-cmd): New test.

* test/lisp/eshell/em-extpipe-tests.el (em-extpipe-test-13): Use
'eshell-concat'.

* doc/misc/eshell.texi (Expansion): Document this behavior.

* etc/NEWS: Announce the change (bug#55236).