cmExtraEclipseCDT4Generator: Replace RunCommand with RunSingleCommand

Avoid use of the old cmSystemTools::RunCommand by using RunSingleCommand
instead.
cmake: Drop "cmake -E comspec" Win9x helper

Since we dropped Win9x support there is no need for this helper.
exec_program: Re-implement using KWSys Process

Drop use of cmSystemTools::RunCommand.  It used popen on UNIX
(equivalent to /bin/sh -c "$command") and direct CreateProcess calls on
Windows.  Implement equivalent behavior using the KWSys Process library.
Copy windows shortpath conversion logic from cmSystemTools::RunCommand.
cmSystemTools: Drop old RunCommand method

All calls to this method have been replaced by newer infrastructure.
Remove it and the supporting cmWin32ProcessExecution class.
Merge topic 'remove-old-process-execution' into next

765d783 cmSystemTools: Drop old RunCommand method
52b80b2 exec_program: Re-implement using KWSys Process
c076476 cmake: Drop "cmake -E comspec" Win9x helper
f551135 cmExtraEclipseCDT4Generator: Replace RunCommand with RunSingleCommand