Add test-coverage of internal function sf_parse_int()

Only the special cases get special handling.

With sanitizers it becomes too easy to trigger a timeout because of the
debug.priv_perf() invocations.

Note for later, I'm planning to change the feature syntax to this:

    feature [!]<name> [<args>...]

This way we can use `feature !sanitizer` to express the same check.

A side effect is slightly more accurate error messages.

Closes #3555

This migrates the remaining candidates for vcc_Peek*() functions using a
coccinelle semantic patch. When vcc_PeekToken*() is used in an assignment
statement it's straightforward to patch. In an expression a careful
review is needed, that's why the generated code is meant not to compile
on purpose. That's how the pattern for vcc_PrintTokens() was noticed.

The same pattern occurred 3 times while reviewing potential usage for the
new vcc_PeekToken*() functions.

Without advancing to the next token, and leaving the door open to
synthetic tokens that would otherwise interfere with direct access.

This is as far declaring symbols goes, because we can still find
hard-coded prefixes in other places. Having them in the types could
actually help centralize them for good, but this is a minimal change
for the purpose of referencing global symbols before they are defined.

Refs #3555

Spotted by: @dridi

(analog of storage_malloc.c)

The buf_len field is only ever modified by the poll thread and should not
need to be guarded here. This will hopefully silence a coverity warning:

   CID 1485138:  Concurrent data access violations  (ATOMICITY)
   Using an unreliable value of "l" inside the second locked
   section. If the data that "l" depends on was changed by
   another thread, this use might be incorrect.

It might on the other hand complain even harder, I have an alternative
if that happens.

Unlike other threads we cancel in varnishtest there's 2 per tunnel and
they need coordination between each other. The spec thread remains as-is
and the poll thread now checks vtc_stop after each condwait.

With this I'm no longer able to reproduce a21's timeout.

Reverts d4cf677a

The assertion above wasn't enough apparently.

And make a note that we should avoid pthread_cancel(3) in the future.

A wrong WRONG doesn't make it right for SunCC.

To cut down synchronization needs for the very beginning of a session.

The tunnel acts like a client and a server and forwards bytes between
two parties. It can then pause in the middle of a session and control
how much data can go in either direction. Combined with barriers it can
be used to trigger socket timeouts, possibly in the middle of protocol
frames.

A tunnel works with two threads: one to execute its spec and one to poll
both parties and transfer data accordingly. The two threads synchronize
via basic pthread primitives and follow a very simplistic state machine:

    ACCEPT -> RUNNING <-> PAUSED -> SPEC DONE -> POLL DONE -> STOPPED

Despite being simplistic, the coordination of two threads would likely
complicate the implementation of a `-repeat` action, but it is still
possible to `start` a tunnel again as shown in the c106 test case that
exercises basic coverage involving a varnish instance.

Usage is documented in the vtc(7) manual.