lib: rework printfrr extensions to output directly

lib: rework printfrr extensions to output directly

Allowing printfrr extensions to directly write to the output buffer has
a few advantages:
- there is no arbitrary length limit imposed (previously 64)
- the output doesn't need to be copied another time
- the extension can directly use bprintfrr() to put together pieces

The downside is that the theoretical length (regardless of available
buffer space) must be computed correctly.

Extended unit tests to test these paths a bit more thoroughly.

Signed-off-by: David Lamparter <equinox@diac24.net>