Pcomplete: Better obey `completion-at-point-functions`

Pcomplete: Better obey `completion-at-point-functions`

Functions on `completion-at-point-functions` should not modify
the buffer.  Pcomplete itself mostly abides by this but Eshell's
use of it doesn't.  Try and catch those cases.  Also fix one of
those cases.

* lisp/pcomplete.el (pcomplete-allow-modifications): New var.
(pcomplete-completions-at-point): Enforce it.
(pcomplete, pcomplete-expand-and-complete, pcomplete-expand):
Rebind it since these commands expect the extra side effects.

* lisp/eshell/em-cmpl.el (eshell--pcomplete-insert-tab): New function,
extracted from `eshell-complete-parse-arguments`.
(eshell-complete-parse-arguments): Use it and obey
`pcomplete-allow-modifications`.