e.g. <undef> >= 0.1 should fail for being an invalid comparison

Conflicts:
	bin/varnishtest/tests/v00013.vtc

Fixes: #1378

The streaming fetch of an object can fail between lookup time and when
we actually use it. Relax this assertion.

Fixes: #1647

Useful with upcoming std function.

Discussed with ingvar.

This avoids false negatives with slow name resolutions

Fixes #1636

The vmod packages need this to know if they are
installable or not for this Varnish version.

Leaving it enabled could hide panics.

Fixes #1639

Conflicts:
	bin/varnishtest/tests/v00026.vtc

Conflicts:
	bin/varnishtest/tests/m00018.vtc

This check never worked as intended for several reasons:

- Didn't take the existing file size into account, causing it to
  succeed on first startup but fail on next

- Sparse files were not handled, and can't be handled as we don't know
  where in the file a hole could be and whether truncation actually
  would change the number of blocks used at all

Fixes: #1343

Improve consistency, add argument names and rewording.

Conflicts:
	lib/libvmod_std/vmod.vcc

Do not advance the subject but use the ovector information as offset,
pcre might need to peek back when handling lookbehinds.

Original patch from MegaMaddin via github.

Fixes: #1557

per object, in line with the functionality as it was in Varnish 3.0.

Conflicts:
	bin/varnishd/cache/cache_vrt_var.c