op.c: Correct comment about OPf_SPECIAL & parens
op.h: Note chdir’s use of OPf_SPECIAL
Fix deparsing of chdir without args

I just broke that in 0175f038c5.  I did not realise that OPf_SPECIAL
means two different things on chdir.  (See the previous commit.)
deparse-skips.t: chdir.t passes
deparse-skips.txt: More passing tests