New backend type: Success. Always returns 200.
* src/nullsink.c: New file.
* src/Makefile.am (libpound_a_SOURCES): Add new source.
* src/capbio.h (BIO_f_sink, bio_new_sink): New prototypes.
* src/config.c (service_parsetab): New keyword: Success.
* src/http.c: Rearrange functions. Provide a central "backend
driver" facility. Implement new internal type of backends: a
"success" backend that always returns 200.
(all special backends): Drain the request content before proceeding.
* src/metrics.c (metrics_response): Likewise.
* src/pound.h (backend_type_str, backend_to_string)
(backend_specific_serialize): New protos.
(str_be): Rewrite as a wrapper over backend_to_string.
(backend_specific_serialize, addrinfo_serialize)
(drain_request, metrics_response
* src/svc.c (backend_serialize): Use backend_specific_serialize to
serializee backend internals.
* tests/success.at: New file.
* tests/Makefile.am: Add new test.
* tests/testsuite.at: Likewise.
* NEWS: Document changes.
* doc/pound.8: Likewise.
* doc/pound.texi: Likewise.
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