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.

Generalize a bit the flow. Expect a request to be written and a body to be sended with net_send_ui. This will make easier to add other protocols like finger or gopher in the future. Also, switch to evbuffer instead of manually calling tls_read/write: with evbuffers we can abstract over the actual I/O routine and just read/write buffers in memory. This too will help with adding more protocols.