msgget.2, semget.2, shmget.2: The purpose of "flags" == 0 is to obtain ID of an exist...

msgget.2, semget.2, shmget.2: The purpose of "flags" == 0 is to obtain ID of an existing IPC object

This was implied in these pages, but the meaning of "flags" == 0
could be more explicit, as indicated by questions such as
https://stackoverflow.com/questions/49833569/flag-value-of-semget-function

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>