33911: turn off ERRFLAG_INT for always block.

Restore bit thereafter: we probably need a new variable in order
to allow user interrupts to be reset in the always block.
33918: add TRY_BLOCK_INTERRUPT

This works the same as TRY_BLOCK_ERROR, but for a SIGINT, too.

Evena added documentation, like the professionals!  Amended along the
lines suggested by Bart in 33918.
fix typo