s3:smbd: convert file_struct.posix_open to a bitmap with flags

s3:smbd: convert file_struct.posix_open to a bitmap with flags

This is in preperation of a more fine grained control of POSIX behaviour
in the SMB and VFS layers.

Inititally we use an uint8_t for the flags bitmap and add a define
posix_flags as posix_open in order to avoid breaking the VFS ABI.

Bug: https://bugzilla.samba.org/show_bug.cgi?id=11065

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>