Backtrace improvements for utilities

[asadsa92]

This PR adds a default back-trace to all asserts outside of varnishd, e.g., varnishstat.
Also add back-trace to varnishtest asserts.

Example (artificially crafted asserts):

---

Before:

Assert error in do_once_cb_first(), varnishstat.c line 164:
  Condition((0) != 0) not true.
Aborted

After:

Assert error in do_once_cb_first(), varnishstat.c line 164:
  Condition((0) != 0) not true.
Backtrace:
  ip=0x7f3458bb6125 sp=0x7fff606013e0 <VBT_format+0x15>
  ip=0x7f3458bb66eb sp=0x7fff60601400 <VBT_dump+0x6b>
  ip=0x7f3458bb5394 sp=0x7fff60601450 <vas_default+0x1b4>
  ip=0x7f3458bb51d3 sp=0x7fff606024c0 <VAS_Fail+0x63>
  ip=0x5614ef51548a sp=0x7fff60602500 <do_once_cb_first+0x3a>
  ip=0x7f3458b963d3 sp=0x7fff60602540 <vsc_iter_seg+0x1c3>
  ip=0x7f3458b957bb sp=0x7fff60602590 <VSC_Iter+0x34b>
  ip=0x5614ef51514e sp=0x7fff60602610 <do_once+0xde>
  ip=0x5614ef514a8d sp=0x7fff60602650 <main+0x5ed>
  ip=0x7f345887124a sp=0x7fff606026c0 <__libc_init_first+0x8a>
  ip=0x7f3458871305 sp=0x7fff60602760 <__libc_start_main+0x85>
  ip=0x5614ef5143d1 sp=0x7fff606027b0 <_start+0x21>
Aborted

---

Before:

*    top   RESETTING after bin/varnishtest/tests/p00000.vtc
**** dT    0.472
**   s1    Waiting for server (5/-1)
**   v1    Wait
**** dT    0.474
---- v1    Assert error in varnish_ask_cli(), vtc_varnish.c line 110:  Condition((0) != 0) not true.  Errno=0 Success
*    top   failure during reset
#    top  TEST bin/varnishtest/tests/p00000.vtc FAILED (0.474) exit=2

After

*    top   RESETTING after bin/varnishtest/tests/p00000.vtc
**** dT    0.472
**   s1    Waiting for server (5/-1)
**   v1    Wait
**** dT    0.474
---- v1    Assert error in varnish_ask_cli(), vtc_varnish.c line 110:  Condition((0) != 0) not true.  Errno=0 Success
Backtrace:
  ip=0x55f0c01f3845 sp=0x7ffd49fc2480 <VBT_format+0x15>
  ip=0x55f0c01f3e0b sp=0x7ffd49fc24a0 <VBT_dump+0x6b>
  ip=0x55f0c01d453f sp=0x7ffd49fc24f0 <vtc_log_VAS_Fail+0x4f>
  ip=0x7fec618c41bb sp=0x7ffd49fc35c0 <VAS_Fail+0x4b>
  ip=0x55f0c01f043e sp=0x7ffd49fc3600 <varnish_ask_cli+0x3e>
  ip=0x55f0c01ee07d sp=0x7ffd49fc3640 <varnish_wait+0x7d>
  ip=0x55f0c01ed132 sp=0x7ffd49fc3660 <cmd_varnish+0x72>
  ip=0x55f0c01b0ddb sp=0x7ffd49fc36d0 <reset_cmds+0x2b>
  ip=0x55f0c01b0d13 sp=0x7ffd49fc36f0 <fail_out+0xa3>
  ip=0x55f0c01d38df sp=0x7ffd49fc3710 <vtc_logfail+0x3f>
  ip=0x55f0c01d3548 sp=0x7ffd49fc3720 <vtc_fatal+0x278>
  ip=0x55f0c01d4707 sp=0x7ffd49fc3830 <vtc_log_VAS_Fail+0x217>
  ip=0x7fec618c41bb sp=0x7ffd49fc4900 <VAS_Fail+0x4b>
  ip=0x55f0c01f043e sp=0x7ffd49fc4940 <varnish_ask_cli+0x3e>
  ip=0x55f0c01f1460 sp=0x7ffd49fc4980 <varnish_launch+0x1020>
  ip=0x55f0c01efcaa sp=0x7ffd49fc5aa0 <varnish_vclbackend+0xaa>
  ip=0x55f0c01ede7f sp=0x7ffd49fc5b00 <cmd_varnish+0xdbf>
  ip=0x55f0c01b0c43 sp=0x7ffd49fc5b70 <parse_string+0xc43>
  ip=0x55f0c01b12c3 sp=0x7ffd49fc6870 <exec_file+0x4c3>
  ip=0x55f0c01dbc01 sp=0x7ffd49fc68d0 <start_test+0x671>
  ip=0x55f0c01d9ebb sp=0x7ffd49fc7940 <main+0x70b>
  ip=0x7fec6151a24a sp=0x7ffd49fc89b0 <__libc_init_first+0x8a>
  ip=0x7fec6151a305 sp=0x7ffd49fc8a50 <__libc_start_main+0x85>
  ip=0x55f0c01a8eb1 sp=0x7ffd49fc8aa0 <_start+0x21>


*    top   failure during reset
#    top  TEST bin/varnishtest/tests/p00000.vtc FAILED (0.474) exit=2

[dridi]

The second patch is easier to review with the --ignore-all-space option (or ?w=1 query string on github).

[dridi]

bugwash: we should enable back-traces outside of varnishd with some kind of developer mode set in varnishd and exposed to VSM readers with a new Arg segment or something similar.

The developer mode should always be assumed by varnishtest (which is much more than a VSM reader) and as an intermediate measure this could be enabled at configure time and always enabled by autogen.des.

[asadsa92]

Bumping this. I assume this can be merged after a rebase?
@dridi I also assume you meant "developer mode" in varnishd can be done later?

[dridi]

I think @bsdphk insisted that we do not make this the default, that it should be controlled in varnishd but we could fall back to a configure-time option (eg. --enable-vut-bt) until we introduce this developer mode.

So I don't think it was acceptable as-is.