ir_sirc: fixup decoder boilerplate

ir_sirc: fixup decoder boilerplate

The SIRC decoder was written to an older API, and failed to load in a
recent environment (tag missing, ambiguous annotation names). Unbreak
the decoder's boilerplate, and address other nits while we are here:
There is no Python output. Use the same declaration syntax as in other
decoders for improved maintenance (greppability). Call reset() from
__init__() to avoid surprises when a future version does have vars. Do
provide a doc string in the module initialization, to keep the URL for
the protocol description at hand.