Address string creation helpers should validate format

[m-ronnblom]

Current xcm_addr_make_() functions for TCP/IP-based transports allow the application to pass an arbitrary string in the form of a purported DNS hostname. The size is limited as per DNS standards, but no other validation is performed.

XCM should validate that the resulting string of a xcm_addr_make_() call is a valid XCM address, and return an error (EINVAL) otherwise. The API already allows for this error, so you can argue this is a backward compatible change.

Also, the documentation should spell out if the "name" field in the struct xcm_addr_host struct may contain an IP address instead of a hostname (which should be allowed, for convenience), when it is passed to a xcm_addr_make_() function.