BUG/MINOR: logs: fix logsrv leaks on clean exit

BUG/MINOR: logs: fix logsrv leaks on clean exit

Log servers are a real mess because:
  - entries are duplicated using memcpy() without their strings being
    reallocated, which results in these ones not being freeable every
    time.

  - a new field, ring_name, was added in 2.2 by commit 99c453df9
    ("MEDIUM: ring: new section ring to declare custom ring buffers.")
    but it's never initialized during copies, causing the same issue

  - no attempt is made at freeing all that.

Of course, running "haproxy -c" under ASAN quickly notices that and
dumps a core.

This patch adds the missing strdup() and initialization where required,
adds a new free_logsrv() function to cleanly free() such a structure,
calls it from the proxy when iterating over logsrvs instead of silently
leaking their file names and ring names, and adds the same logsrv loop
to the proxy_free_defaults() function so that we don't leak defaults
sections on exit.

It looks a bit entangled, but it comes as a whole because all this stuff
is inter-dependent and was missing.

It's probably preferable not to backport this in the foreseable future
as it may reveal other jokes if some obscure parts continue to memcpy()
the logsrv struct.