bug#15362: 24.1; pcomplete unnecessarily restricts tab-completion entry points

[Vivek Dasmohapatra]

> Have you set completion-cycle-threshold?  If yes and it still doesn't
> cycle, please post a recipe.

With the recipe as before, but _without_ erc-pcomplete at the head
of erc-complete-functions so the value is:

  '(erc-pcompletions-at-point erc-button-next-function)

And with completion-cycle-threshold set to 5, I get the following from 
repeatedly tabbing:

ERC> jlf: jlf_``: jlindsay: jlf: jlf_``: jlindsay: jlf:

so instead of cycling it's just adding the next element from the 
matching candidates list each time.