There is a scan logic issue where the main libclamav scanning functions
create an extra "nested" fmap for each file being scanned. This is
slightly inefficient for a normal scan, but causes a major performance
issue when using ENGINE_OPTIONS_FORCE_TO_DISK. It causes every scanned
file to be duplicated in the temp directory before the scan.

We fix this by using `cli_magic_scan()` in `scan_common()` instead
of `cli_magic_scan_nested_fmap_type()`. We can do this now that the
`cl_scandesc_callback()` API creates an fmap for the caller, instead of
the old logic where `scan_common()` called different API's depending on
whether or not we have an fmap or a file descriptor.