Prefer g_seekable_seek() over camel_stream_reset().

Prefer g_seekable_seek() over camel_stream_reset().

When a stream is obviously a file or memory stream (both of which
implement the GSeekable interface), use g_seekable_seek() instead of
camel_stream_reset().

This is helping me discover if it's safe to remove camel_stream_reset().
We want to eventually move to GIO streams, which have no reset method.