Adjust built-in payload transmit buffer size

The built-in payload transmit buffer is used for Login Accepted, Login
Rejected, End of Session, Login Request and Logout Request packets. Set
the size of the buffer to the size of the largest of these, the Login
Request packet.
Make built-in receive buffer size configurable

The maximum payload length is 65534 bytes. This is much larger than the
maximum message length of many application protocols. Make the built-in
receive buffer size configurable.
Merge pull request #4 from jvirtanen/features/soupbintcp-buffer-size

Adjust SoupBinTCP buffer size