Previously in case when evbuffer_reserve_space() returns > 1, but
it was able to read only 1 IO vector, it will try to read the next one,
got 0 (EOF for mbedTLS or SSL_ERROR_ZERO_RETURN for OpenSSL) and will
trigger EOF, while instead, it should trigger EV_READ w/o EOF and only
after EOF.
This patch splits common part out to avoid copy-paste from the
- bufferevent_openssl.c
- bufferevent_mbedtls.c
It uses VFS/bufferevent-like approach, i.e. structure of callbacks.