X-Debbugs-Cc: monnier@iro.umontreal.ca

Eshell lets you put quotes around variable names so that the parser can 
tell where the name ends, sort of like ${var} in other shells:

   ~ $ echo $'user-login-name'-suffix
   user-suffix

   ~ $ echo $"user-login-name"-suffix
   user-suffix

However, you can't tab-complete variable names when you do this. Here's 
a fix. I also fixed a couple small issues with completing directory 
names where it would sometimes complete to "whatever/ ". That extra 
trailing space isn't helpful, since you'd have to delete it before 
typing in a subdir.

Probably the most controversial part of this patch is in #0002, where I 
added another dynamic variable 'pcomplete-exit-function' that Pcomplete 
handlers can set to tell Pcomplete what to do after exiting a 
completion. Maybe it would be better to have handlers throw some special 
value for 'pcomplete-completions' that contains this info (sort of like 
the value that a 'completion-at-point-function' returns). I'm not sure 
what the best (and most-compatible) way to do this would be...