detect Varnish instances and try to be helpful

[gquintard]

One Karnaugh map later and I realized that #116 (comment)
could have been expressed more simply:

• as a rule, when we issue at least one warning, we pause for 10s to give time to the user to read said warning
• if we can ping varnish, proceed
• if we can't ping:
  • if -n was given: warn then proceed
  • if there's no -n:
    • without any candidate, warn then proceed
    • there's only one candidate, and we can ping it: override, warn then proceed
    • there's only one candidate but we can't ping: error out
    • more than one candidate: list and error our

Read on for example outputs.

If varnishadm ping is successful with the current NAME argument, no
question asked, go for it:

Info: Working directory: /tmp/varnishgather.Z1LTcYva/varnishgather-flamp-20241106-214646
Info: Complete varnishadm command line deduced to:
Task: 1: varnishd -V
...

If pinging failed, there are a few cases:

If NAME was given, or if NAME wasn't given but there's no varnishd
detected, issue a warning, pause a bit, then proceed:

Info: Working directory: /tmp/varnishgather.BBE414gf/varnishgather-flamp-20241106-221201
#######################################################
using: varnishadm    ping,
We couldn't ping varnishd, Is it running and is
./varnishgather running with the right permissions?
#######################################################
Info: Complete varnishadm command line deduced to:
<PAUSE 10s>
Task: 1: varnishd -V
...

if no NAME was given but there's exactly one candidate, and it's
pingable, override NAME, warn about it, pause then proceed:

Info: Working directory: /tmp/varnishgather.ZeUDcfg7/varnishgather-flamp-20241106-222151
#######################################################
using: varnishadm    ping,
We couldn't ping varnishd, Is it running and is
./varnishgather running with the right permissions?

There's only one varnishd running and it's pingable.

Overriding -n argument to /tmp/beepboop
#######################################################
<PAUSE 10s>
Info: Complete varnishadm command line deduced to: -n /tmp/beepboop
Task: 1: varnishd -V
...

If no NAME is given, and there's only one candidate but it's not
pingable, just error out:

Info: Working directory: /tmp/varnishgather.eZSa78VU/varnishgather-flamp-20241106-224856
#######################################################
using: varnishadm    ping,
We couldn't ping varnishd, Is it running and is
./varnishgather running with the right permissions?
#######################################################
<EXIT>

Finally, if there are no given NAME, and we have more than one
candidate, we just propose a list of candidates and error out:

Info: Working directory: /tmp/varnishgather.dkGgfqHe/varnishgather-flamp-20241106-223210
#######################################################
using: varnishadm    ping,
We couldn't ping varnishd, Is it running and is
./varnishgather running with the right permissions?

We've also detected one or more varnishd that
could possibly be pinged with these -n arguments:
				-n /tmp/beepboop
				-n /tmp/beepboop2
#######################################################
<EXIT>

@hermunn and @zackmay, tagging you in as you were part of the original discussion.

[gquintard]

@simonvik , @hermunn , @dridi , merging at the end of the week unless someone objects

[simonvik]

I dont have any objections, it seems to work but is a bit unreadable :) (but so is the rest of the code)