This function sets the maximum record packet size in this
connection. This property can only be set to clients. The server
may choose not to accept the requested size.
Acceptable values are 512(=2^9), 1024(=2^10), 2048(=2^11) and
4096(=2^12). The requested record size does get in effect
immediately only while sending data. The receive part will take
effect after a successful handshake.
This function uses a TLS extension called 'max record size'. Not
all TLS implementations use or even understand this extension.
On success, GNUTLS_E_SUCCESS (zero) is returned,
otherwise an error code is returned.