value-pairs: Value pairs functions should honour time zone mode.

Time zone mode specifies if we should use the local timezone or the send
timezone when formatting time related macros. Value-pairs foreach/walk
functions also can format time related macros, so we should propagate
the time zone mode down to these functions, instead of wiring LTZ_LOCAL.

Signed-off-by: Viktor Tusa <tusa@balabit.hu>