cmSystemTools: Teach RunSingleCommand to merge child pipes when possible

Audit the code to make sure there are no callers that use OUTPUT_MERGE
with separate capture strings.  Then change RunSingleCommand to
implement output merging by giving the child process a single pipe for
both its stdout and stderr descriptors.  This will more cleanly merge
the content on atomic write boundaries in the child instead of on
arbitrary buffering boundaries in the parent.