Eliminate almost all backward jumps by replacing this common pattern:

int
some_random_function(void)
{
    int result = 0;
    ...

  cleanup:
    <unconditional cleanup code>
    return result;

  failure:
    <cleanup code in case of an error>
    result = -1;
    goto cleanup
}

with this simpler pattern:

int
some_random_function(void)
{
    int result = -1;
    ...
    result = 0;

  cleanup:
    if (result < 0) {
        <cleanup code in case of an error>
    }

    <unconditional cleanup code>
    return result;
}

Add a bool success variable in functions that don't have a int result
that can be used for the new pattern.

Also remove some unnecessary memsets in error paths.