Make 'eshell-bol' obsolete

Now that Eshell uses fields for its output, 'eshell-bol' is no longer
needed, and we can just use 'beginning-of-line'.

* lisp/eshell/esh-mode.el (eshell-bol): Mark obsolete.
(eshell-mode-map): Remove 'C-a' mapping.
(eshell-command-map): Use 'move-beginning-of-line'.
(eshell-move-argument, eshell-kill-input): Use 'beginning-of-line'.
(eshell-get-old-input): Remove unnecessary call to
'eshell-skip-prompt-function'.

* lisp/eshell/em-rebind.el (eshell-rebind-keys-alist): Remove 'C-a'
and '<home>' mappings; the global mapping for these
('move-beginning-of-line') does the same thing now.

* lisp/eshell/em-cmpl.el (eshell-complete-parse-arguments):
* lisp/eshell/em-elecslash.el (eshell-electric-forward-slash):
* lisp/eshell/em-hist.el (eshell-hist-word-reference)
(eshell-previous-matching-input-from-input, eshell-test-imatch):
* lisp/eshell/em-prompt.el (eshell-backward-matching-input):
* lisp/eshell/em-rebind.el (eshell-point-within-input-p):
* test/lisp/eshell/eshell-tests.el (eshell-test/forward-arg): Use
'beginning-of-line'.

* test/lisp/eshell/eshell-tests.el (eshell-test/run-old-command):
Rename to...
(eshell-test/get-old-input): ... this, and expand the test.

lisp/eshell/em-cmpl.el

@@ -312,7 +312,7 @@ to writing a completion function."
 	     (eshell-interactive-process-p))
     (eshell--pcomplete-insert-tab))
   (let ((end (point-marker))
-	(begin (save-excursion (eshell-bol) (point)))
+	(begin (save-excursion (beginning-of-line) (point)))
 	(posns (list t))
 	args delim)
     (when (and pcomplete-allow-modifications