SHELL_ArgifyW().
- Move "%I" expansion from ShellExecuteEx() into common function
  SHELL_ArgifyW().
- Pass buffer length to SHELL_FindExecutable().
- FIXME comment for len paramater in SHELL_ArgifyW().

Ge van Geldorp <ge@gse.nl>
- Add double quotation marks unless we already have them (e.g.: "%1"
  %* for exefile).
- Remove unnecessary double quotation marks and command line arguments.