In this patch I am trying to get rid of the Irrelevant types from the
signatures of the functions from the standard C library. For that I've
introduced lookupType() to be able to lookup arbitrary types in the global
scope. This makes it possible to define the signatures precisely.
Note 1) fread's signature is now fixed to have the proper FILE *restrict
type when C99 is the language.
Note 2) There are still existing Irrelevant types, but they are all from
POSIX. I am planning to address those together with the missing POSIX functions