bug#56509: 29.0.50; Eshell variable aliases don't fully support indexing

[Jim Porter <jporterbugs@gmail.com>]

Normally in Eshell, you can index variables to get the nth element, e.g. 
"echo $exec-path[0]" would print the first element in your `exec-path' 
variable. However, this doesn't always work for "alias" variables. From 
"emacs -Q -f eshell":

   ~ $ alias first 'echo $1[0]'

   ~ $ first (list 1 2)
   (1 2)

This should return 1, since it says to print the first (well, 0th) 
element of the first argument.

There's also a related issue with alias variables that reference other 
alias variables:

   ~ $ echo $PWD
   /home/jim

   ~ $ echo $+
   /home/jim

   ~ $ cd ..

   /home $ echo $PWD
   /home

   /home $ echo $+
   /home/jim

The '$+' alias is mapped to '$PWD', but rather than using Eshell's 
definition for '$PWD', it erroneously consults the system's '$PWD' 
environment variable, which in this case is the directory that Emacs was 
started from.

Patch forthcoming; just getting a bug number.