Lock down global variables in if_tap (primarily, the tap softc list); add tapmtx, which protects globale variables.
Lock down global variables in if_tap (primarily, the tap softc list);
add tapmtx, which protects globale variables.

Notes:

- The EBUSY check in MOD_UNLOAD may be subject to a race.  Moving the
  event handler unregister inside the mutex grab may prevent that race.

- Locking of global variables safely is now possible because tapclones
  is only modified when the module is loading or unloading, thanks to
  phk's recent chang to clone_setup().

- softc locking to follow.

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