Revert 7c35db07c35db0... msiexec: Use CommandLineToArgvW instead of process_args to reduce code duplication.

The previous command line processing is not identical to
CommandLineToArgvW and CommandLineToArgvW breaks msi command line
processing for things such as Excel Viewer 2003.

Specifically the command line created by the Excel Viewer 2003 and
passed to msiexec.exe is "c:\windows\system32\msiexec.exe" /I
C:\windows\temp\IXP000.TMP\XLVIEW.MSI CDCACHE="2"
LAUNCHEDFROMSETUP="1" SETUPEXEPATH="C:\windows\temp\IXP000.TMP\"
SETUPEXENAME="SETUP.EXE" /lpiwaeo "C:\windows\temp\Microsoft Office
Excel Viewer 2003 Setup(0001)_Task(0001).txt" CDCACH"...