As I was relying on the lack of -E option to discard another type of
non-ESI sub-requests I recently misled myself with this assumption. It
took me a while to realize that I was also seeing transactions I was not
interested in, corrupting the statistics I was deriving from a log dump.

Now that vmod_pesi [1] sets the precedent, we should expect new types of
sub-requests to materialize and expecting the reason for the transaction
to be "esi" is no longer appropriate.

We could also track this field as vtx::sub_level for example, to make it
available as VSL query LHS operand. The focus here was to adjust the
behavior of an existing feature, not add a new one.

[1] https://code.uplex.de/uplex-varnish/libvdp-pesi