i18n: fix "% inside _()" problems
Before this patch, "contrib/check-code.py" can't detect these
problems, because the regexp pattern to detect "% inside _()" doesn't
suppose the case that format string consists of multiple string
components concatenated implicitly or explicitly,
This patch does below for that regexp pattern to detect "% inside _()"
problems in such case.
- put "+" into separator part ("[ \t\n]") for explicit concatenation
("...." + "...." style)
- enclose "component and separator" part by "(?:....)+" for
concatenation itself ("...." "...." or "...." + "....")