permissions: properly merge propagation flag

permissions: properly merge propagation flag

when multiple roles are defined on a path that share a privilege, this
randomly took the propagation flag for the priv from the last role
encountered. since perl hashes are iterated randomly, this means the
propagation flag was sometimes set correctly, and sometimes not.

note that this propagation flag is only used for display/dumping
purposes, and for intersection with token privs (see next commit).
actual handling of propagation happens on the role level in
PVE::AccessControl::roles().

modified test case (spuriously) fails without the fix.

Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>