Switch to 32 bit frame limit

I'm pretty sure it was broken anyway.
I haven't ever tested by sending a frame over 4GB
in length.  Anywho, for now, this will stay 32 bit.
You can still receive *messages* greater than 32 bits
as long as the fragments that compose them are not
greater than 32 bits in size.

Also, fix configure.ac for older autotools.
Playing around with struct evbuffer_iovec usage.