treewide: unify ENABLE_* defines
Two styles were used: (un)defined and 0/1. We switch to 0/1.
Advantage: it can be used also like if (ENABLE_FOO)
(outside preprocessor).
Except for ./meson.build it's
sed \
-e 's/#ifdef ENABLE_CAP_NG/#if ENABLE_CAP_NG/g' \
-e 's/#ifdef ENABLE_DOH2/#if ENABLE_DOH2/g' \
-e 's/defined(ENABLE_COOKIES)/ENABLE_COOKIES/g' \
-e 's/#ifdef ENABLE_COOKIES/#if ENABLE_COOKIES/g' \
-i $(git grep -l ENABLE_)