Dynamically complete the command at point.
This function is similar to `comint-dynamic-complete-filename', except that it
searches `exec-path' (minus the trailing Emacs library path) for completion
candidates. Note that this may not be the same as the shell's idea of the
path.
Completion is dependent on the value of `shell-completion-execonly', plus
those that effect file completion.
Returns t if successful.
(fn)