GN_Esa, Read & write to filename segments

IN:
     A = manipulator command
          A7:  1 = write, 0 read
          A0:  1 = extension, 0 = name
     HL/DE = filename and buffer pointers

     A7 = 0 (reading): HL = filename, DE = buffer
     A7 = 1 (writing): HL = new segment, DE = buffer

     B = segment number (+/- 64)
     Positive indexes start at the device name (B=0). Negative indexes start
     at the filename (B=255,-1).

     C = Limit of buffer space to use (only relevant for writing). C should
     not be zero.

A7 = 0 (reading):OUT, if call successful:
     Fc = 0
OUT, if call failed:
     Fc = 1
     A = error code:
          RC_IVF ($17), bad filename

Registers changed after return:
     ..BCDEHL/IXIY same
     AF....../.... different

A7 = 1 (writing):OUT, if call successful:
     Fc = 0
     B = number of filename segments returned
     C = number of characters returned
     DE = indexes terminating null of filename

OUT, if call failed:
     Fc = 1

Registers changed after return:
     ......HL/IXIY same
     AFBCDE../.... different