lib: Add SAFE_KILL()

Signed-off-by: Xiaoguang Wang <wangxg.fnst@cn.fujitsu.com>
sendfile/sendfile06.c: cleanup and fix some bugs

If we run sendfile06 with "-i large number" option, we may get
EMFILE error, this is because we don't close the 'sockfd' and
'out_fd' file descriptor in do_sendfile(), fix this bug.

We also modify the code in do_child(), let child do not loop
'TEST_LOOPING(lc)'times. This makes no sense, and if we run with
"-i large number", this will slow down the program.

Delete some useless comments.
Use SAFE_* macros.
Some cleanup.

Signed-off-by: Xiaoguang Wang <wangxg.fnst@cn.fujitsu.com>
Signed-off-by: Cyril Hrubis <chrubis@suse.cz>