Fix occasional merged sections in unpacked crash report

rich-core-extract sometimes puts sections near the end of the rich core into
a single file. This commit provides new, shorter and hopefully more readable
implementation of the extraction functions that doesn't suffer from the
mentioned deficiency.

Tested on variety of collected rich cores.
Handle special case of first section's header in Quickie reports

Allow to omit initial newline of RICHCORE_HEADER at the beginning of
input file. Quick Feedback sends such reports and strictly speaking they
are malformed, but old rich-core-extract accepted the format and we
already have quite a number of those quickies collected, so let's
promote the short header to feature.