input: clear sdi_ready flag and receive() buffer in common code

input: clear sdi_ready flag and receive() buffer in common code

Rephrase common input reset logic such that additional common code can
execute after the individual module's reset callback. No behaviour has
changed, the module's reset callback still is optional, and identical
log output gets emitted.

Do clear the sdi_ready flag in the common sr_input_reset() routine, so
that all input modules will parse header information again before
processing sample data when subsequent calls receive new file content.
Void the input module's receive() buffer from common reset code. This
unbreaks the feature of re-reading previously consumed input files.

Extend comments in the common reset and free code paths, which involve
the modules' reset and cleanup routines, which interact in non-trivial
ways. Discuss the responsibilities of common and individual routines, to
remain aware during maintenance.