If non-nil, hitting the TAB key cycles through the completion list.
Typical Emacs behavior is to complete as much as possible, then pause
waiting for further input. Then if TAB is hit again, show a list of
possible completions. When `pcomplete-cycle-completions' is non-nil,
it acts more like zsh or 4nt, showing the first maximal match first,
followed by any further matches on each subsequent pressing of the TAB
key. M-x pcomplete-list is the key to press if the user wants to see
the list of possible completions.