Rename most 'package' to 'message'.

Rename most 'package' to 'message'.

RADIUS (RFC2865) is defined to be transported over UDP so the term
"radius packet" makes a lot of sense. RADIUS/TCP (RFC6613) and
RADIUS/TLS (RFC6614), a.k.a. RadSec, use stream transport protocols
though. The term "message" doesn't imply any kind of transport -- a
message can be sent using datagrams as well as in a stream.

This (large) commit changes 'package' to 'message' where it makes
sense. It does not touch the 'radius' subdirectory. It includes
preprocessor directives (#define) to make the public interface
compatible with previous releases of the library.