This simply replaces the use of VRT_synth_strands() with
VRT_synth_blob(). That's more of a workaround than an explanation
of the problem.

Closes #2

In file included from vmod_file.c:44:
vmod_file.c: In function ‘check’:
/usr/include/varnish/cache/cache.h:66:26: error: format not a string literal and no format arguments [-Werror=format-security]
   66 | #define NO_VXID ((struct vxids){0})
      |                          ^~~~~
vmod_file.c:171:32: note: in expansion of macro ‘NO_VXID’
  171 |                 VSL(SLT_Error, NO_VXID, errbuf);
      |                                ^~~~~~~
      ...

The PRIV_VCL list is now a list of reader objects. On the warm event,
we restart the timer and then wait for the first check to run, as
in object init.

Rather than contain it entirely, in the continued effort for
better cacheline-friendliness. This is a considerable improvement,
but we're still not quite there.

The update check works on copies of the info struct, flags, etc.
Only take the write lock when all work is finished, then copy
data back under the lock.

Can't rule out error messages longer than 256 chars.

Still very cacheline-unfriendly.

(Not before.)

Fixes #1

The thought was to shorten the time spent under the lock. But it
actually introduces a race.