Add ability for Eshell virtual targets to handle closing the target

Add ability for Eshell virtual targets to handle closing the target

This was documented to work by calling the output function with 'nil',
but that was never actually implemented.  Instead, for compatibility,
we now support a new (optional) close function.

* lisp/eshell/esh-io.el (eshell-virtual-targets): Update docstring.
(eshell-generic-target): New struct...
(eshell-function-target): ... inherit from it, and rename from
'eshell-virtual-target'.
(eshell-get-target): Handle already-created 'eshell-generic-target'.
(eshell-close-target): Call the target's close function if present.

* test/lisp/eshell/esh-io-tests.el (esh-io-test/virtual/device-close):
New test.

* doc/misc/eshell.texi (Redirection): Document the new behavior.