Add API for transport-level attributes

[m-ronnblom]

Occasionally a user program may want to know some property of the transport without having an actual socket available. For example, it may be useful to know the

• The maximum size of a message
• The type (messaging, or byte stream)

int xcm_tp_attr_get(const char *tp_name, const char *attr_name, enum xcm_attr_type *type, void *value, size_t capacity);

One use case may be a program wanting to give an early error indication (rather than waiting for an actual socket to be created).