feat(dracut): add a DRACUT_EXTRA_ARGS environment variable

feat(dracut): add a DRACUT_EXTRA_ARGS environment variable

Some build tools (e.g. autopkgtest-build-qemu) invoke dracut indirectly
through update-initramfs via dpkg triggers, making it impossible to pass
extra flags directly on the command line.

Add a DRACUT_EXTRA_ARGS environment variable whose contents are
word-split and prepended to the dracut argument list before option
parsing.  This allows callers to inject arbitrary flags without modifying
the configuration on disk:

  DRACUT_EXTRA_ARGS=--no-hostonly autopkgtest-build-qemu ...

The variable is intentionally word-split (like MAKEFLAGS), so arguments
that themselves contain spaces must be shell-quoted within the value:

  DRACUT_EXTRA_ARGS='--add "mod1 mod2"' dracut

Changed test 13 to provide test coverage for DRACUT_EXTRA_ARGS variable.