Fix bug #6421 - POSIX read-only open fails on read-only shares.

The change to smbd/trans2.c opens up
SETFILEINFO calls to POSIX_OPEN only. The change to first smbd/open.c closes 2
holes that would have been exposed by allowing POSIX_OPENS on readonly shares,
and their ability to set arbitrary flags permutations. The O_CREAT ->
O_CREAT|O_EXCL change removes an illegal combination (O_EXCL without O_CREAT)
that previously was being passed down to the open syscall.
Jeremy.