BUG/MINOR: log: fix global lf_expr node options behavior (2nd try)

BUG/MINOR: log: fix global lf_expr node options behavior (2nd try)

In 98b44e8 ("BUG/MINOR: log: fix global lf_expr node options behavior"),
I properly restored global node options behavior for when encoding is
not used, however the fix is not optimal when encoding is involved:

Indeed, encoding logic in sess_build_logline() relies on global node
options to know if encoding must be handled expression-wide or
individually. However, because of the above fix, if an expression is
made of 1 or multiple nodes that all set an encoding option manually
(without '%o'), we consider that the option was set globally, but
that's probably not what the user intended. Instead we should only
evaluate global options from '%o', so that it remains possible to
skip global encoding when needed.

No backport needed.