Explicitly state the number of arguments a hook callback should have.

[David Hampton]

Log Message:
-----------
Explicitly state the number of arguments a hook callback should have.
Scheme gets very unhappy if you don't supply the right number of
arguments.

Tags:
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gnucash-gnome2-dev

Modified Files:
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    gnucash/src/engine:
        gnc-hooks.c
        gnc-hooks.h
        gw-engine-spec.scm