options: allow logging and debugging to be configured by environment variables

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Some logging, like that in rtdl, should be controlled by environment variables (so that we don't have to recompile the program to enable it).

#598 implements this for malloc et al., but we ought to do this in a way that is safe w.r.t concurrent modifications of the environment. For example, we could have a global libcConfig struct that we initialise in ld.so from the environment at program startup.