MINOR: arg: Add an argument type to keep a reference on opaque data

MINOR: arg: Add an argument type to keep a reference on opaque data

The ARGT_PTR argument type may now be used to keep a reference to opaque data in
the argument array used by sample fetches and converters. It is a generic way to
point on data. I guess it could be used for some other arguments, like proxy,
server, map or stick-table.

(cherry picked from commit e02fc4d0dd57c92bbe96f3ff2ae0b890405458f2)
[wt: needed by next commit]
Signed-off-by: Willy Tarreau <w@1wt.eu>
(cherry picked from commit bf1fcab0670c3a94270d19c50a23a88b3d0edfba)
Signed-off-by: Willy Tarreau <w@1wt.eu>