GN_Pfs, Parse correctness of filename segment

IN:
     HL = 0,1,2; not allowed (B=0)
     BHL = points to filename segment
     (B = 0 is interpreted as local pointer)

OUT, if call successful:
     Filename syntactically correct
     Fc = 0
     A0 = 1, extension used
     A1 = 1, filename used
     A2 = 1, explicit directory used
     A3 = 1, current directory (".") used
     A4 = 1, parent directory ("..") used
     A5 = 1, wildcard directory ("//") used
     A6 = 1, device specified
     A7 = 1, wildcards used

     BHL = points to terminating character

OUT, if call failed:
     Fc = 1
     A = error code:
          RC_IVF ($17), poor syntax
          RC_EOF ($09), blank segment

Registers changed after return:
     ..BCDE../IXIY same
     AF....HL/.... different