Implement getenv, putenv etc.

[robdockins]

PR #290 implemented a dummy version of getenv that simply always returns NULL, which simulates an empty collection of environment variables.

We should instead provide a realistic implementation of getenv that, at a minimum, provides an immutable set of environment variables that can be provided during simulator setup.

Ideally, we would also allow a full implementation of setenv, etc. However, doing this properly requires would require additional support for symbolic string values, and probably special support from the LLVM memory model for handling null-terminated symbolic strings.