Protect cache against future expansions of FcLangSet (adding new
orth files).  Previously, doing so could change the size of
that struct.  Indeed, that happened between 2.6.0 and 2.7.3, causing
crashes.  Unfortunately, sizeof(FcLangSet) was not checked in fcarch.c.

This changes FcLangSet code to be able to cope with struct size changes.
And change cache format, hence bumping from 2 to 3.