hash: fix FLAGS_CREATE_FILE O_WRONLY preventing pread on new hash table

hash: fix FLAGS_CREATE_FILE O_WRONLY preventing pread on new hash table

FLAGS_CREATE_FILE used O_WRONLY|O_CREAT|O_EXCL.  On a fresh beeshome,
open_file() creates beeshash.dat through this flag and assigns the
write-only fd to m_fd.  The prefetch thread then calls pread_or_die(m_fd)
and gets EBADF because pread() requires O_RDONLY or O_RDWR.

In production, beesd and OpenRC pre-create beeshash.dat with truncate(1)
before exec'ing bees, so the creation branch in open_file() was never
exercised.  Integration tests exposed this by starting bees on a fresh
$BEESHOME.

Change FLAGS_CREATE_FILE from O_WRONLY to O_RDWR.  The name still
describes the intent (O_CREAT|O_EXCL ensure exclusive creation); O_RDWR
simply allows reading the file back after writing it.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>