Simplify parsing subcommands slightly

Simplify parsing subcommands slightly

This mainly reduces some overly-deep indentation, but also fixes some
minor issues with the "$<subcmd>" form: it unnecessarily added " >
TEMP" (we already set this later via 'eshell-create-handles'), and it
didn't properly unescape inner double quotes.

* lisp/eshell/esh-cmd.el (eshell-parse-subcommand-argument): Simplify.

* lisp/eshell/esh-var.el (eshell-parse-variable-ref): Simplify and
fix edge cases in "$<subcmd>".

* test/lisp/eshell/esh-var-tests.el
(esh-var-test/quoted-interp-temp-cmd): Adjust test to check behavior
of inner double quotes.