nginx-util: fix ubus::~iterator() and minor issues

* Do not destroy the iterator twice if cur==this (segfault).
* Do not add the delimiter clim=='\0' when creating the SSL directives.
* Set the right SSL_SESSION_CACHE_ARG for nginx-util get_env.
* Remove static from the constexpr that are used only for Line::build.
* Concat strings instead of appending them for not using a non-const ref
(to remove some warnings of clang-tidy -checks=google-runtime-references)

Signed-off-by: Peter Stadler <peter.stadler@student.uibk.ac.at>