Add visibility tags
Remove unused method
Don't check for unused header files
Don't check for "standard" headerfiles

There is no point of search for header files listed in the
language spec (like stdlib.h etc)
Don't expose the inner workings of the SASL library

The clients shouldn't touch the internals of the library
and shouldn't know what it looks like.
Remove file IO from the library

The library should operate on an inmemory password database
to avoid dependencies towards other fileformats (and to avoid
race conditions when the file is updated or not)