system call about files

implement the
int sys_open(char*);
int sys_filesize(int fd);
int sys_tell(int fd);
int sys_close(int fd);
int sys_read(int fd, void* buffer, off_t size);
int sys_create(char* file, off_t size);
int sys_seek(int fd, off_t pos);
int sys_write(int fd, const void* buffer, off_t size);
int sys_remove(char *file);