BUG/MINOR: tools: do not create an empty arg from trailing spaces

BUG/MINOR: tools: do not create an empty arg from trailing spaces

Trailing spaces on the lines of the config file create an empty arg
which makes it complicated to detect really empty args. Let's first
address this. Note that it is not user-visible but prevents from
fixing user-visible issues. No backport is needed.

The initial issue was introduced with this fix that already tried to
address it:

    8a6767d266 ("BUG/MINOR: config: don't count trailing spaces as empty arg (v2)")

The current patch properly addresses leading and trailing spaces by
only counting arguments if non-lws chars were found on the line. LWS
do not cause a transition to a new arg anymore but they complete the
current one. The whole new code relies on a state machine to detect
when to create an arg (!in_arg->in_arg), and when to close the current
arg. A special care was taken for word expansion in the form of
"${ARGS[*]}" which still continue to emit individual arguments past
the first LWS. This example works fine:

    ARGS="100 check inter 1000"
    server name 192.168.1."${ARGS[*]}"

It properly results in 6 args:

    "server", "name", "192.168.1.100", "check", "inter", "1000"

This fix should not have any visible user impact and is a bit tricky,
so it's best not to backport it, at least for a while.

Co-authored-by: Valentine Krasnobaeva <vkrasnobaeva@haproxy.com>