comint loses prompt boundary

["Ilya N. Golubev" <gin@mo.msk.ru>]

`comint.el' xemacs packages revision: 1.19.  emacs version - one
bundled with 21.4a.

`inhibit-field-text-motion' is `t', `comint-use-prompt-regexp' is
`nil', if it helps to reproduce.

In shell mode buffer with text already output and entered

user@host:~> COMMAND

(comint-bol nil)

does not skip past the prompt, as documented.  Instead, it goes to the
beginning of line.  `shell-backward-command' calls it, and goes to the
same location.  `shell-dynamic-complete-command' calls it to obtain
position of text already entered by user and incorrectly decides not
to try to complete it as command.

User may set up editor so that interactive generic text motion
commands (including `beginning-of-line', `forward-paragraph') notice
or not notice fields.  Regardless of this
(`inhibit-field-text-motion') setting, expecting commands explicitly
intended for comint mode buffers to distinguish between commands and
prompts properly.  Let alone always expecting command completion to
work.

It is always possible for comint to maintain, in addition to field
text properties, another ones of its own, specifically to distinguish
process outputs and user inputs.  After all, if it depends on text
fields that much, its commands can set `inhibit-field-text-motion'
temporarily (bind dynamically) to the value they expect.