Simplify d/rules

Remove unused $DEB_HOST_MULTIARCH
Don't use --disable-silent-rules
Use d/clean to remove generated files
Remove unused Depends
Fix invalid-short-name-in-dep5-copyright
Add --fail-missing in dh_install
Tweak debian/control
Do not include libchewing.a in libchewing3-dev
Bump standard version to 3.9.6
Install include into /usr/include/triplet
Update debian/changelog