LibC: Change putenv (and getenv) to not copy, but directly return the environ values.

This is in keeping with how putenv should function. It does mean that
the shell's export command now leaks, but that's not a difficult fix.

Contributes to #29.

LibC/stdio.cpp

@@ -293,6 +293,8 @@ void rewind(FILE* stream)
 
 int dbgprintf(const char* fmt, ...)
 {
+    // if this fails, you're printing too early.
+    ASSERT(stddbg);
     va_list ap;
     va_start(ap, fmt);
     int ret = vfprintf(stddbg, fmt, ap);