More precise warnings about function returns

This commit introduces a control-flow static analysis over C abstract syntax (file cparser/Cflow.ml) and uses it to
- warn for non-void functions that can return by falling through the body
- warn more precisely for _Noreturn functions that can return
- introduce the "return 0" in "main" functions less often (cosmetic).

For the control-flow analysis, the following conservative approximations are made:
- any "goto" label is reachable
- all cases of a "switch" statement are reachable as soon as the "switch" is reachable (i.e. the switch expression takes all values needed to reach every case)
- the boolean expressions in "if", "while", "do"-"while" and "for" can take true and false values, unless they are compile-time constants.

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