When a Decimal operation raises multiple signals and more than one of
those signals is trapped, the specification determines the order in
which the signals should be handled.  In many cases this order wasn't
being followed, leading to the wrong Python exception being raised.
This commit fixes those cases, and adds extra tests.  The tests are
only enabled when EXTENDEDERRORTESTS is True, since they involve
rerunning each Decimal testcase several times.

_io module doesn't call this function which was introduced in Python3.

Found using Clang's static analyzer.

Found using Clang's static analyzer.

Was not applied to modules that will not compile under OS X, dbmmodule.c,
getaddrinfo.c, and getnameinfo.c.

Found using Clang's static analyzer.

passed to free() and make a `char *` to a `const char *` as found by Clang's
static analyzer.

Found using Clang's static analyzer.

Found using Clang's static analyzer.

Found with Clang's static analyzer.

Found using Clang's static analyzer.

Found using Clang's static analyzer.

Found using Clang's static analyzer.

Found using Clang's static analyzer.

Clang.

Found using Clang's static analyzer.

Found using Clang's static analyzer.

exceptions raised by the implicit flush().  Also ensure that calling
close() several times is supported.  Patch by Pascal Chambon.

gc.DEBUG_STATS.

uses the default browser.

works on OSX

(yes, this is a sensitive subject, sorry)

test_multiprocessing.  Patch by Paul Moore.

Only override the AC_PROG_CC determined CFLAGS if they were set by the user.
This restores the default behavior in the common case of not having CFLAGS
defined when running configure.

Mac/README. Fixes issue 7107.