Update Qt 4.8.3 download link

Conflicts:
	contrib/gitian-descriptors/README
misc small spelling/indentation fixes
Merge commit 'efde29e' into 0.8.x
Fix and improve release-process.md

This commit adds a step, which is to git checkout the version to be
built. This ensures that the gitian-descriptors for said version will be
the correct ones. In addition, it updates the links for several
dependencies, where the previously existing links were dead.

Conflicts:
	doc/release-process.md
fix #3049 by changing comment for CNetAddr::IsRFC4193()

Conflicts:
	src/netbase.h
Merge commit '71888bf' into 0.8.x

Conflicts:
	src/netbase.h