Receive multiple file descriptors with a Request

Extending the existing code to support receiving more than one file
descriptor per request. The micro-http crate might be used in a context
where multiple file descriptors are associated with one request, hence
the need to update the micro-http crate.

A concrete example from Cloud Hypervisor is to be able to pass multiple
TAP file descriptors at once when adding a new network interface. This
way it can hotplug a multiqueue device.

Signed-off-by: Sebastien Boeuf <sebastien.boeuf@intel.com>

coverage_config.json

@@ -1 +1 @@
-{"coverage_score": 93.1, "exclude_path": "", "crate_features": ""}
+{"coverage_score": 93.2, "exclude_path": "", "crate_features": ""}