Unsafe code calling setenv after fork

When passing new environment veriables to a child process, we call setenv after forking - but this is unsafe and may lock up (if some other thread holds a malloc lock at the time of forking).

We should instead build a new envp array prior to the fork, and pass it to execvpe

In practice this is only (currently) used from eclccserver and we have not observed it causing any issues.