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_)