input: add confidence (detection strength) to format_match()

input: add confidence (detection strength) to format_match()

When users don't specify the input format, applications can try to have
the format auto-detected. Some of the tests were weak and could result
in false positives.

Add a 'confidence' parameter to the input modules' format_match()
method. Claim high confidence (1) for those formats which can check
magic strings or the presence of essential keywords (vcd, wav). Claim
medium confidence (10) for those formats which happened to mis-detect
inputs in the past (trace32_ad). Claim weak confidence (100) for those
formats which totally lack reliable conditions and rather guess than
detect (chronovu_la8).

Prefer the best match in public scan routines. Return at most one module
to callers even if multiple modules matched, to keep the current API.

This addresses part of bug #1200.