Fix bug #6421 - POSIX read-only open fails on read-only shares. The change to smbd...

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.
(cherry picked from commit 79f26472b4ae561ec00c30f31dd63ccab6dfc0c4)