ignore access mode bits of flags in mkostemps and functions that use it

per the text accepted for inclusion in POSIX, behavior is unspecified
when any of the access mode bits are set. since it's impossible to
consistently report this usage error (O_RDONLY could not be detected
since its value happens to be zero), the most consistent way to handle
them is just to ignore them.

previously, if a caller erroneously passed O_WRONLY, the resulting
access mode would be O_WRONLY|O_RDWR, which has the value 3, and this
resulted in a file descriptor which rejects both read and write
attempts when it is subsequently used.