Processes created by the worker might require a TMPDIR for temporary
files (think: filters). Create a dedicated directory for this purpose,
which is otherwise hard to achieve in a universal manner.

The problem is that a MAP_FAILED segment ends up in its vsm_set::segs
list, at which point NULL checks will determine what to do with the
segment, and MAP_FAILED != NULL.

This can happen when winning (or losing?) the race with a VSC segment
that goes away after being found in the index, but before being mapped.

One surefire result is a crash in vsm_unmapseg() tripping on assertions
that other fields are also not NULL. There may be other unidentified ill
effects.

It should be resolved like the argument of a -n option.

Refs 1d7cdadd

Putting the new fallback next to the historical fallback.

Refs #4105

The host name default referenced by VUT_OPT_n is long gone since
879c9b37

Refer to varnishd(1) to avoid the documentation getting out of sync.

Ref #4105

with greetings from the rabbit-hole opened by #4105

to anchor patches

Maybe it should gain a boc_state_e * argument to return a consistent
state value?

It turns out we still need it in a couple places.

When TMPDIR is much longer than /tmp, it becomes easy to run out of
abuf. Since abuf and pbuf are initially used to hold an IP address
and a port number, it becomes explicit in their declaration. Likewise
the new lbuf may hold a file name and safely be reused for CLI auth
and other purposes.

The test is racy, we should always perform the backend fetch to reach
the "barrier b1 sync" in the server body, otherwise the test will be
stuck on the second barrier sync until timeout.

Fixes: #4098