SECURITY: XSS in langconverter when regex hits pcre.backtrack_limit

SECURITY: XSS in langconverter when regex hits pcre.backtrack_limit

Adjust regexes for what not to convert to avoid backtracking by
preferring possesive quantifiers

Add check that we really have matched to the end of the string, and
log error if the regex hits some sort of error preventing the
entire string from being matched. Should the regex not match to the
end, then language conversion is disabled for the string.

Bug: T124404
Change-Id: I4f0c171c7da804e9c1508ef1f59556665a318f6a