Simplify handling of /dev/null redirection in Eshell

Simplify handling of /dev/null redirection in Eshell

This also fixes an issue where "echo hi > foo > /dev/null" didn't
write to the file "foo".  (Note that users can still use their
system's null device name when redirecting; Eshell doesn't need to do
anything special to support that.)

* lisp/eshell/esh-io.el (eshell-virtual-targets): Add "/dev/null".
(eshell-set-output-handle): Handle 'eshell-null-device'.

* test/lisp/eshell/esh-io-tests.el
(esh-io-test/redirect-subcommands/dev-null)
(esh-io-test/virtual/dev-null, esh-io-test/virtual/dev-null/multiple):
New tests.