Changeset c8fcf66 in git

src/avenprcore.cc,src/message.c,src/message.h: msg() has returned a
persistent copy of the message for decades, so the code's bound to
rely on that in some places. And there's no real reason it
shouldn't continue to as the messages are under 20KB of UTF-8 text
for any language, so lazy loading isn't worth the complication. So
remove msgPerm() and the unused msgFree() macro, and document that
msg() returns a persistent copy.