MEDIUM: log: lf_* build helpers now take a ctx argument

MEDIUM: log: lf_* build helpers now take a ctx argument

Add internal lf_buildctx struct that is only used inside
sess_build_logline() scope and is passed to lf_* log building helpers
to expose current building context. For now, node options and the in_text
counter are stored in the ctx struct. Thanks to this change, lf_* building
functions don't depend on a logformat_node struct pointer, and may be used
in a standalone manner as long as a build context is provided.

Also, global options are now handled explictly in sess_build_logline() to
make sure that global options are always considered even if they were not
duplicated on every nodes.

No functional change should be expected.