change type of fileDomain to (must-set, may-set)
fixes for warning about unclosed files at the end
-> keep sets of unclosed files as special value in domain
fix warnings for multiple returns: all tests pass
two warnings are issued (must and may), but only the last one (may) is correct
TODO: warn when leaving main and not at return
cleanup
fix error-branch for fopen, add option ana.file.optimistic
added extra state Error since some state for join is needed and Closed isn't right