Improve handling of local variable settings in Eshell

Improve handling of local variable settings in Eshell

This ensures that these commands work the same as normal commands,
aside from making environment variable settings local to that command.
Among other things, this means that "VAR=value cd dir/" now changes
the directory correctly.

* lisp/eshell/esh-var.el (eshell-in-local-scope-p)
(eshell-local-variable-bindings): New variables.
(eshell-var-initialize, eshell-set-variable): Use
'eshell-local-variable-bindings'.
(eshell-handle-local-variables): Don't use 'eshell-as-subcommand'.

* test/lisp/eshell/esh-var-tests.el (esh-var-test/local-variables/cd):
New test.