scheck/php: support for variadic arguments

Summary: @override-unit-failures

Test Plan:
unit tests included (for parsing and scheck)
make tests

Reviewers: eletuchy, glevi, julienv, jwatzman

Reviewed By: jwatzman

Differential Revision: https://phabricator.fb.com/D1528610
Merge branch 'master' of github.com:facebook/pfff