And generally bring debian/copyright up to date.

Functions returning a heap-allocated string should return a "char *",
not a "const char *": Because the result is heap-allocated and becomes
the responsibility of the caller, it is certainly modifiable.  And the
pointer will likely get passed to free(), triggering a conversion
warning from gcc.  So remove all the relevant consts.

Windows doesn't generally use POSIX error codes, which poses a problem
for librabbitmq's approach of using those error codes in its API.  So
make the librabbitmq error codes opaque: They are still be integers,
but client code is not supposed to assume anything about them, except
that they can be passed to a new amqp_error_string() function which
returns the corresponding error message

Internally, the error codes are either taken from a set of
librabbitmq-specific values, or correspond to an OS-specific (POSIX or
win32) error code, with a simple encoding to indicate which is which.