find: Correctly propagate -exec/-execdir ... {} + exit status.

As per POSIX, the -exec ... {} + primary always returns true, but a non-zero
exit status causes find to return a non-zero exit status itself. GNU does
the same, and also for -execdir ... {} +.

It does not make much sense to return false from the primary only when the
child process happens to be run.

The behaviour for -exec/-execdir ... ; remains unchanged: the primary
returns true or false depending on the exit status, and find's exit status
is unaffected.