PROOF: check if single ds before tokenizing name

The specified datasets string is checked to see if it corresponds to
one dataset only. If not, it is broken into pieces (with "|," as
separators) and each token is considered a separated dataset name.

This should fix an issue where a single dataset name might contain
commas.
Minor PROOF cleanups
PROOF: check for unparsed AliEn dataset names

Unparsed portions in data specification trigger an error. Also, some
generic functions have been declared static.
PROOF: fixed issue with multi-ds and entry list