Allow makeaclitem() to accept multiple privilege names.

Allow makeaclitem() to accept multiple privilege names.

Interpret its privileges argument as a comma-separated list of
privilege names, as in has_table_privilege and other functions.
This is actually net less code, since the support routine to
parse that already exists, and we can drop convert_priv_string()
which had no other use-case.

Robins Tharakan

Discussion: https://postgr.es/m/e5a05dc54ba64408b3dd260171c1abaf@EX13D05UWC001.ant.amazon.com