Some internal code treats the message ids as encoded "variable length
quantities", while other internal code assumes the message id is
always one byte long. Continue using this scheme, but convert the VLQ
users to use the name "msgtag" while the 1-byte users use "msgid".
Increase the number of available msgids from 96 to 127 - the higher
values get encoded as negative "msgtags".
Signed-off-by: Kevin O'Connor <kevin@koconnor.net>