Add support for GNU AGPLv3 license (AGPL-3)

It was verified that a generated license file is identical to the
original copy at http://www.gnu.org/licenses/agpl-3.0.html.
Merge pull request #1361 from tarleb/agpl

Add support for GNU AGPLv3 license (AGPL-3)
80-col violation.
Merge remote-tracking branch 'upstream/master'
Use soft constraints for add-source deps in sandboxInstallPolicy.

See #1362. This is an experimental change.
Comment.
Merge remote-tracking branch 'origin/master'
Merge remote-tracking branch 'upstream/master'
Make 'removeBuildTreeRefs' tolerate non-existing paths.

Fixes #1360.
Clarify what the arguments of 'mappendSomeSavedFlags' are.

Also add 'configProgramArgs' and 'configProgramPaths' to the set of
special-cased flags.
Merge remote-tracking branch 'upstream/master'