We must ensure that we get a complete header name before evaluating
a ban, so "req.http." should not be accepted.

Refs #3962

Ban expression variables that partially match standard variable names
(ex: req.urlXX) should not be accepted, except for variables that take
an HTTP header name as a suffix.

Fixes #3962

by default, add "User-Agent: cNAME" header to all client txreq requests
if not already present (via -hdr). This can be disabled with -nouseragent

by default, add a "Server: sNAME" header to all server txresp responses
if not already present (via -hdr). adding -noserver to txresp will
disable it. Note that for -dispatch servers, the id of the spawned server
(sN) will be used instead of s0.

Fixes #3960

Suppose the following happens:

vbp_task() finishes with vt->running = 0 and a heap insert. The
vbp_cond is signaled under the lock, but now instead of vbp_thread()
waking up first, VBP_Remove() gets the lock and reaches
assert(vt->heap_idx == VBH_NOIDX) before the racing vbp_thread()
deleted the heap.

This is unlikely to happen with static backends, because for those,
the probe is stopped via the vcl temperature before they get removed.

Fixes https://github.com/nigoroll/libvmod-dynamic/issues/100

And mark it for flexelint

my local config on linux is slightly different than phks on freebsd

Inside the loop the element is in fact, not null. Only one occurrence
was caught by check_obj.cocci, which shows that more code is still not
understood by Coccinelle. For example, it doesn't seem to enjoy #undef
directives inside functions.

I get a new warning with Coccinelle XXX:

> magic, previously declared as a metavariable, is used as an identifier

I don't remember seeing this warning before, but it looks like @@ blocks
may not behave as I thought. I didn't review the other semantic patches.

Ref: 582ded6a

Fixes #3958

Fixes: left operand must be modifiable lvalue: op "="

Same as: 64f7181b

This leaves exactly one commit after the coccinelle efforts.
What a waste of time. :|

someone(tm) should have checked the history and found
9fd79a7c

This should make them stand out more, and help revisit them more
frequently.

Avoiding conflicts with #3954.

Inspired by #3954.

Ref #3948

FreeBSD/arm (not 64) vtester

**** p2    stderr|Error: Child failed on launch within cli_timeout=3.00s (tip: set startup_timeout)
**** p2    stderr|Error: Child (29805) said Child starts
**** p2    stderr|Error: Child (29805) said -sdebug init delay 0.000000s
**** p2    stderr|Error: Child (29805) said -sdebug open delay in init 5.000000s
**** p2    stderr|Error: Child (29805) died signal=6
**** p2    stderr|Error: Child (29805) said Wrong turn in cli_quit(), ../../../../bin/varnishd/cache/cache_main.c line 371: pthread_kill(cli_thread, sig) failed
**** p2    stderr|Error: Child (29805) said   errno = 22 (Invalid argument)
**** p2    stderr|Error: Child (29805) said Wrong turn in child_signal_handler(), ../../../../bin/varnishd/cache/cache_main.c line 327: Signal 6 (Abort trap) received at 0x0 si_code 65543
**** p2    stderr|Error: Child (29805) said   errno = 22 (Invalid argument)

Some systems see it, others not