Use build.d everywhere and take arguments by parameter ...

It seems easier to forward arguments from the build script this way,
and it's much more common to change the build arguments than the version.

Add support for DragonFlyBSD ...

- Switch from /bin/sh to /usr/bin/env bash
- Ignore temporary and test-build files (gitignore)
- Untrack changes to source/dub/version_.d
- Handling 'isDragonflybsd'
- Use 'tr -d' instead of 'tr --deleted' which works on Linux and BSD based platforms
- Fix FRONTEND = unbound variable (test/run-unittest.sh)

Fix the RPM package build script.

Rename "installer" folder to "scripts" to broaden the scope of it.