magic.h: Present message labels as an encoding of major and minor numbers.

The encoding is strange for historical reasons, but represents all pairs
of 16-bit major and minor codes.

I've exhaustively verified that the encoding is invertable, and that it
reproduces the old manually assigned labels; this program is
`msgcode-test.c', which I've added to the standard test run, though it's
rather slow to run.

Signed-off-by: Mark Wooding <mdw@distorted.org.uk>