Add config_intrhook_oneshot(): schedule an intrhook function and unregister
it automatically after it runs.

The config_intrhook mechanism allows a driver to stall the boot process
until device(s) required for booting are available, by not allowing system
inits to proceed until all intrhook functions have been unregistered.
Virtually all existing code simply unregisters from within the hook function
when it gets called.

This new function makes that common usage more convenient. Instead of
allocating and filling in a struct, passing it to a function that might (in
theory) fail, and checking the return code, now a driver can simply call
this cannot-fail routine, passing just the intrhook function and its arg.

Differential Revision: https://reviews.freebsd.org/D11963