MINOR: action: new '(http-request|tcp-request content) do-resolve' action

MINOR: action: new '(http-request|tcp-request content) do-resolve' action

The 'do-resolve' action is an http-request or tcp-request content action
which allows to run DNS resolution at run time in HAProxy.
The name to be resolved can be picked up in the request sent by the
client and the result of the resolution is stored in a variable.
The time the resolution is being performed, the request is on pause.
If the resolution can't provide a suitable result, then the variable
will be empty. It's up to the admin to take decisions based on this
statement (return 503 to prevent loops).

Read carefully the documentation concerning this feature, to ensure your
setup is secure and safe to be used in production.

This patch creates a global counter to track various errors reported by
the action 'do-resolve'.