lib: record output argument positions in zlog

lib: record output argument positions in zlog

printfrr() recently acquired the capability to record start/end of
formatting outputs.  Make use of this in the zlog code so logging
targets have access to this information.

(This also records how long the `[XXXXX-XXXXX][EC 9999999]` prefix was
so log targets can choose to skip over it.)

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