bug#57129: 29.0.50; Improve behavior of conditionals in Eshell

[Jim Porter]

In Eshell, you can use conditionals pretty much like you'd expect from 
other shells. Starting from 'emacs -Q -f eshell':

  ~ $ [ foo = foo ] && echo hi
  hi
  ~ $ [ foo = bar ] && echo hi
  ~ $ [ foo = foo ] || echo hi
  ~ $ [ foo = bar ] || echo hi
  hi
  ~ $ if {[ foo = foo ]} {echo yes} {echo no}
  yes
  ~ $ if {[ foo = bar ]} {echo yes} {echo no}
  no

However, that only works for external commands. If the command is 
implemented in Lisp, it doesn't work:

  ~ $ (zerop 0) && echo hi
  t
  hi
  ~ $ (zerop 1) && echo hi
  hi  ;; Shouldn't say hi.

That's because the exit status is always 0 for Lisp commands. This works 
correctly for external commands:

  ~ $ [ foo = foo ]; echo status $? result $$
  ("status" 0 "result" nil)
  ~ $ [ foo = bar ]; echo status $? result $$
  ("status" 1 "result" nil)

But not for Lisp commands:

  ~ $ (zerop 0); echo status $? result $$
  t
  ("status" 0 "result" t)
  ~ $ (zerop 1); echo status $? result $$
  ("status" 0 "result" nil)
  ~ $ (zerop "foo"); echo status $? result $$
  Wrong type argument: number-or-marker-p, "foo"
  ("status" 0 "result" nil)

The manual says that the status should be 1 when a Lisp command fails, 
but it's 0 no matter what, even if it signaled an error. (Likewise, the 
manual says that the "result" variable should be t for successful 
external commands, but it's always nil.)

Similarly to the above, you can't use variable expansions for conditionals:

  ~ $ (setq foo t)
  t
  ~ $ if $foo {echo yes} {echo no}
  yes
  ~ $ (setq foo nil)
  ~ $ if $foo {echo yes} {echo no}
  yes  ;; Should say no.

Patch forthcoming. Just splitting it into two messages since it seemed 
more readable that way...