Info-list operations should use 'list' as the parameter rather than 'ptr'

[abouteiller]

Overview

While working on #492 (comment)
we found that the name ptr for the list parameter in the PMIx_Info_list_get_info was unclear. However after review we found that this ptr problem is pervasive and should be tackled all at once.

Motivation

Clarification of parameter intent is always good.

Discussion

Mirror change should be made to Open PMIx and the ABI document.