Fix bug with fseek(..., SEEK_CUR) on read-only streams.

This would seek to the wrong location, and in some cases try to seek before the beginning of the file, leading to an error condition.

The problem stemmed from fseek calling fflush, which sets up the stream's buffer state in a way appropriate for writing but not for reading. It then calls ftell, which (for a read-only stream) would misinterpret this state and miscalculate the current position.

The fix is to make fflush work correctly on a read-only stream, setting up the state for reading. (User code calling fflush on a read-only stream would be undefined behavior, but since fseek does it we need to make it work.)

This fixes #6.

stdio.asm

@@ -4484,7 +4484,7 @@ orVal    ds    2                        for setting the case of characters
          iny
          txa
          sta   [stream],Y
-         ldy   #FILE_base                 set the end of buffer mark
+         ldy   #FILE_base               set the end of buffer mark
          lda   [stream],Y
          ldy   #FILE_size
          clc
@@ -4500,13 +4500,20 @@ orVal    ds    2                        for setting the case of characters
          dey
          dey
          sta   [stream],Y
-         ldy   #FILE_size                 set the number of chars the buffer
+         ldy   #FILE_flag               if read stream
+         lda   [stream],Y
+         bit   _IOREAD
+         beq   ib1
+         lda   #0                         set count of chars in buffer to 0
+         tax
+         bra   ib2                      else
+ib1      ldy   #FILE_size                 set the number of chars the buffer
          lda   [stream],Y                  can hold
          tax
          iny
          iny
          lda   [stream],Y
-         ldy   #FILE_cnt+2
+ib2      ldy   #FILE_cnt+2
          sta   [stream],Y
          dey
          dey