Support large messages on TLS, TCP, UXS and USTLS transports

[m-ronnblom]

Add support for large (1MB) messages on TLS and TLS and the imaginary (at the time of writing) UXS and USTLS transports.

The idea is to make the "xcm.msg_msg_size" writable at socket creation, and to keep the old (constant) value of 65536 bytes as the default. This is useful since some applications may already use this value to size stack allocations. Such applications will get an unpleasant surprise if this number grows something very much larger (potentially larger than the calling thread's stack).

A configurable max message size may also be useful to optimize memory management in the transport, and for applications to have XCM drop messages larger than allowed by the application layer protocol (running on top of XCM).

Introduce a new attribute "xcm.max_max_msg_size", to communicate what is the max configurable limit for the message size ceiling.

No other API changes should be required.

No wire-protocol changes are required.