New subfunctions for syscall 58

APPEND uses byte level access, because 512 bytes block handling feels little silly. It return error code if file not exist or start position is outside file.

Code:

fileinfo:                 ; fileinfo for function 58
  .mode        dd 3       ; 3=append file
  .start_block dd -1      ; StartPos in bytes 0..File Size (-1 = EOF)
  .blocks      dd 0x0     ; bytes to write (0 = truncate file at StartPos)
  .address     dd buffer  ; source data pointer
  .workarea    dd 0x10000 ; work area for os
  dirnamez:    times 128 db 0

fileinfo.mode = 3  ; append file  (return: ebx = bytes written)
fileinfo.mode = 12 ; get_filesize (return: ebx = file/dir size in bytes)
fileinfo.mode = 13 ; get_fileattr (return: ebx = file/dir attribute)
fileinfo.mode = 14 ; get_filedate (return: ebx = file/dir date/time)