Unify build-script running to always use rpmfcExec()

[pmatilai]

Simplifies the code and could be of use to PR #593

The downside is that it eats more memory due to buffering the entire build output but whether that matters... and could be conditionalized of course.

[ffesti]

I wonder if we want something even more generic that allows to pass stdin, stdout and stderr as stringbuffers and fdup also both stdout and stderr.
It would allow passing NULL for any of these. That way we would not use more memory if not needed.

[pmatilai]

Yeah I thought about the same thing, passing NULL as the buffer should mean not to buffer it at all.
Capturing stderr requires bigger changes though, as getOutputFrom() is only equipped to grab stdout now.

So this doesn't really make sense as it is now.

[pmatilai]

Merged in slightly different form as part of #670