Settings that globally affect the behavior of Async.
These can be set by setting an environment variable, ASYNC_CONFIG, to a sexp
representation of the config. Also, setting ASYNC_CONFIG to an invalid sexp
(e.g. the empty string), will cause your program to print to stderr a usage message
describing how to configure ASYNC_CONFIG, and exit nonzero. For example, the
following shell command should print the usage message:
ASYNC_CONFIG= foo.exe
Documentation on these is in strings in config.ml, so it can be output in the help message.