The current code is not able to unset an environment variable, only to set it to an empty value. This patch refactors a bit the Message class and uses a new EnvChanger helper class that allows to change temporarily an environment variable and that is able to unset variables if needed.
The patch also adds new functions hasEnv and unsetEnv in environment.cpp.
Open issues:
* there may be systems where unsetenv is not available and putenv("name=") does not do the right thing;
* unsetenv may lead to leaks on some platforms.
* when using unsetenv, we may need to remove strings from the internal map that setEnv uses.
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