BUG/MINOR: rules: Forbid captures in defaults section if used by a backend

BUG/MINOR: rules: Forbid captures in defaults section if used by a backend

Captures must only be defined in proxies with the frontend capabilities or
in defaults sections used by proxies with the frontend capabilities. Thus,
an extra check is added to be sure a defaults section defining a capture
will never be references by a backend.

Note that in this case, only named captures in "tcp-request content" or
"http-request" rules are possible. It is not possible in a defaults section
to decalre a capture slot. Not yet at least.

This patch must be backported to 2.5. It is releated to issue #1674.