instead of defining macros in compat.h. It's clearer that it's only for libevent2 this way

it seems that we can "melt" :P the evbuffer from the start to avoid the unfreeze/freeze dance

on libevent2 we need to wrap evbuffer_add with evbuffer_freeze/evbuffer_unfreeze. Not sure exactly why, probably because we're doing some evbuffer_enable/disable/enable-again. Retain compatibility with the custom libevent1 in base on OpenBSD.

sizeof(buf) should be equal to howmuch, otherwise we may end up reading with tls_read out of bounds and corrupting the stack.

avoid a dummy `struct req;'

it's not really required, we're gonna quit anyway, but it's cleaner and helps reducing the clutter when using leak detection tools.