CLEANUP: resolvers: simplify resolv_link_resolution() regarding requesters

CLEANUP: resolvers: simplify resolv_link_resolution() regarding requesters

This function allocates requesters by hand for each and every type. This
is complex and error-prone, and it doesn't even initialize the list part,
leaving dangling pointers that complicate debugging.

This patch introduces a new function resolv_get_requester() that either
returns the current pointer if valid or tries to allocate a new one and
links it to its destination. Then it makes use of it in the function
above to clean it up quite a bit. This allows to remove complicated but
unneeded tests.

(cherry picked from commit 239675e4a955b219e915fa11a1a03c7aacc13ccd)
Signed-off-by: Willy Tarreau <w@1wt.eu>
(cherry picked from commit 4d7ca9149914b1188e117608a96b5608232f0bc5)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>