[runtime] Switch getenv to use heap memory

[runtime] Switch getenv to use heap memory

Getenv doesn't use heap memory, it uses static memory.
Mono's usage is consistent with a view of getenv memory contents
that are durable and can be stored and manipulated.

In a multithreaded context, getenv calls can race and corrupt
string buffers as they are read. In a single threaded context,
subsequent calls to getenv will lead to overwriting the memory
that pointers to previous calls point to.

In order to bring memory safety back, we duplicate memory and
manage it at each call-site.

Note: this changes the API semantics of g_getenv. This is necessary
because the locking around the getenv buffer is safest when confined to
the function in eglib.