MEDIUM: proxy: Warn about ambiguous use of named defaults sections

MEDIUM: proxy: Warn about ambiguous use of named defaults sections

It is now possible to designate the defaults section to use by adding a name
of the corresponding defaults section and referencing it in the desired
proxy section. However, this introduces an ambiguity. This named defaults
section may still be implicitly used by other proxies if it is the last one
defined. In this case for instance:

  default common
    ...

  default frt from common
    ...

  default bck from common
    ...

  frontend fe from frt
    ...

  backend be from bck
    ...

  listen stats
    ...

Here, it is not really obvious the last section will use the 'bck' defaults
section. And it is probably not the expected behaviour. To help users to
properly configure their haproxy, a warning is now emitted if a defaults
section is explicitly AND implicitly used. The configuration manual was
updated accordingly.

Because this patch adds a warning, it should probably not be backported to
2.4. However, if is is backported, it depends on commit "MINOR: proxy:
Introduce proxy flags to replace disabled bitfield".