Ref: c15eefb1

This is already possible with some overhead (for example with a
de-tour via vcl_pass), but I see absolutely no reason why we should
not allow direct restarts from recv.

The use case is to rewrite the request and re-start processing of the
rewritten request as if it was received as such. This is also possible
already with some vcl sub juggling, but is complicated by the VCL call
loop detection. Use of restarts avoids this complication and is safe
due to max_restarts.

just a minor stylistic cleanup: Early returns should mark exceptional
cases.

how many instances of the VSC set there are.

generalize functions and methods, some additional cleanup

This eliminates a O(N) lookup when we want to do something
(ie: Destroy) to a VSC segment subsequently.

...Now that we will need more than one integer.

This ensures we don't end up with a VSL batched record with conflicting
ids, which breaks the -g grouping modes.

Originally it was the plan that the JSON would be gzip'ed in the
VSM, but to avoid exposing VGZ in the API we didn't, but kept the
gzip'ed JSON in the generated VSC code because why not?

Well now we know why not:  It wastes a lot of time for very little space,
in particular if you have many backends.

Provides some relief for: #2513 & #2518

phk probably has this in a global flexelint config

Fixes: #2351

.keys() doesn't produce a list in python3, only an iterable view

fix: #2511

Spotted by ASAN.

VCL_STRANDS is an alternative to VCL_STRING_LIST which uses a
struct instead of varargs.

This means that multiple VCL_STRANDS can be passed to a function,
and string-compare really needed that to not waste workspace.

Add <, >, <=, >= string comparisons while here anyway.

The struct and a const char *[N] array are built on the stack and
they are only valid for the duration of the function call.

Add a new type "STRINGS" which is internal to vcc_expr.c only,
and convert to "STRING" and "STRING_LIST" at the edge of vcc_expr.c
and as necessary inside.  (More to come.)

Various and sundry cleanups along the way.