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.