Add curl_parallel_limit setting that will replace media_fetch_threads in
a later commit.

Fix a typo in MutexedQueue::pop_back() that made it impossible to compile
code that used this function. (Noticed this while implementing httpfetch.)

Similar to assuming a selection box for the nodebox drawtype, minetest.register_item() now assumes a selection box for the fencelike drawtype.

Most modders would otherwise forget to explicitly define this, and generated nodes aliased from mods would wall-off caves