[RFC] a corner case of open(2)

[Al Viro]

	According to POSIX (and behaviour on other Unices) the following
should succeed: open("/tmp", O_CREAT, 0) does not have O_EXCL and the pathname
does refer to existing object, so O_CREAT is ignored and the call is
equivalent to open("/tmp", 0), which succeeds.

	We have it rejected with EISDIR.  The thing is, the standard behaviour
is actually less messy wrt code, and do_last()/lookup_open()/atomic_open()
badly needs untangling.

	Another place where we produce a bogus EISDIR is O_CREAT|O_EXCL on
an existing directory.  POSIX (and other Unices) have EEXIST there.  In some
cases we produce EEXIST, in some - EISDIR.  Uniform EEXIST is actually easier.

	It is a change of user-visible behaviour, but I would be very
surprised if anything broke from that change.  And it would help to simplify
the awful mess we have in there.

	Comments?