[GH #892] more capture fixes

actually fill the provided stdout and stderr arguments to capture.
handle missing stderr.

Fixes also the failing win32 tests where I have no IO::CaptureOutput #1158
[test] fix t/native_pbc/number.t for windows

windows prints 1.12589990684262e+015 and not 1.12589990684262e+15
Fixes 2/3 of #1158