Tidy-up in EvalContextVariablePut().

Split over-long lines, moved some variable declarations closer to
their use.

Comment on the unwisdom of ignoring VariableTablePut()'s return.
Unfortunately, the obvious solution (forward that return value to our
caller, instead of returning true regardless) breaks various tests
(e.g. 01_vars/01_basic/013.cf), which is Not Good.
Only define tests/unit/db_test.c's CreateGarbage when used.

Its uses were subject to #if-ery; so apply similar to its definition.
Silences a warning when it's unused.
Suppress unused parameter warnings in tests/load/.
Tidy-up in db_load.c

Some overly-complex code to do simple things, some stray code
commented out that might as well be removed.  Moved CoinFlip one
function earlier to save the need for pre-declaration.