MEDIUM: debug: make the complain() function print the prefix

MEDIUM: debug: make the complain() function print the prefix

The prefix ("ABORT at", "FATAL:", "WARNING:") as well as the second level
("bug condition", ...) are entirely determined by the details bit field,
so there is no reason to have to build this string in the caller, instead
we can let complain() build it entirely from the details into the local
iovec, so let's do that.

The prefix is now dropped from the whole call chain as it's no longer
useful. This further reduces the code size by 17.5 kB.