Fix facts about Q.positive/negative/etc and Q.real/Q.extended_real (closes #5976)

    Q.positive/Q.negative (etc) are meaningfull now
    for Q.extended_real (== Q.real | Q.infinity).
Fix refine.py to use Q.extended_real
Fix order.py to use Q.extended_real
Fix tests
Fix few inconsistencies: mostly with negative powers of the extended
reals

    e.g.: ask(Q.positive(1/x), Q.positive(x)) is None, but
          ask(Q.positive(1/x), Q.positive(x) & ~Q.infinity(x)) is True
fix more tests (XXX: split this commit)