New shared function, getenv_bool()

This provides a standard way to get a boolean from an environment
variable.  It treats the variable as true iff its value begins with 'y'
or 'Y', like most of the current implementations.  The function takes a
default value which it returns if the environment variable is undefined.

This replaces the various ad-hoc tests of environment variable scattered
around and mostly doesn't change their behaviour.  The exceptions are
TOWERS_2D in Towers and DEBUG_PUZZLES in the Windows front end.  Both of
those were treated as true if they were defined at all, but now follow
the same rules as other boolean environment variables.

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