Specific system calls may benefit from dealing with the caller's
runtime mode by themselves, depending on internal information which
the generic syscall dispatcher does not have access to.

To this end, a new syscall mode called "handover" is
introduced. Syscalls bearing this mode bit are always entered from the
current calling domain. The syscall handler may return -ENOSYS to
trigger a switch to the converse domain until all domains have been
visited once, at which point the syscall fails with -ENOSYS
automatically.