pkgsummary documentation cleanup

Close bug in changelog, and clarify 'special' reporting-sections.
Change pkgsummary 'summ' references to 'summary'

Twas a silly name anyway.
Improve pkgsummary reliability and testability

This mostly means consolidating all 'flag' information into a single
structure. Also, arguments from external calls are validated a bit.
test_pkgsummary.py: Initial pkgsummary tests

The primary goal is to check for and flag changes to the external
summary format used by downstream consumers.
conf.py: Add get_std_distro()

Try to determine the right 'standard' distribution for the section,
either 'oldstable', 'stable', 'testing', 'unstable', or 'experimental',
based on the settings from get_distro() and get_distros().
Returns 'unknown' if undetermined.
piuparts-report.py: Support default reporting-section definition

If reporting-sections is 'default', use conf.get_std_distro() to
determine which distribution to assign the results to. If
reporting-sections is 'none', don't make the section summary.
conf.py: _map_distro properly for '*-proposed'

There was one active section in piuparts.conf.pejacevik that didn't
map properly, squeeze2wheezy-proposed. Strip e.g. '-proposed' from the
distro name before processing.
conf.py: Add _map_distro() support for all others

Map rc-buggy and Debian<x>.<y> to the standard distribution names.
Also, re-enable the standard name tests for sid and experimental -
they are not time sensitive.