Dynamically complete STUB from CANDIDATES list.
This function inserts completion characters at point by completing
STUB from the strings in CANDIDATES. A completions listing may be
shown in a help buffer if completion is ambiguous.
Returns nil if no completion was inserted.
Returns `sole' if completed with the only completion match.
Returns `shortest' if completed with the shortest of the matches.
Returns `partial' if completed as far as possible with the matches.
Returns `listed' if a completion listing was shown.
See also `pcomplete-filename'. (fn STUB CANDIDATES &optional CYCLE-P)