Subtle bug-fix: when executing a native external program, the server process's
environment is no longer modified to set variables in the spawned process's environment. This could result in a race condition where if multinodes executed native externals simultaneously, one node's child process could get the wrong node number (SBBSNNUM) or node directory (SBBSNODE) value (in theory). We now use cool new string list functions to copy and append string blocks to copy the environment of the server before modifying and passing to the child process. This bug-fix only applies to Win32 builds.