use _wsystem() in stead of system() on Windows.

Use g.argv[0] in stead of fossil_nameofexec() when the full path is not necessary, e.g for "Usage" prints.

Minor additional fixes related to [c8c0b78c84]:
- Use _wsystem() in stead of system() on Windows
- Use g.argv[0] in stead of fossil_nameofexe() where only printing is involved
- Use fossil_nameofexe() where fossil_system() is involved
- Move the determination of the full windows path to fossil_nameofexec,
  so g.argv[0] can always be the unmodified argv[0]
- g.argv[0] and fossil_nameofexec() are in utf-8, so we cannot use fprintf
  directly. Exception: when printing malloc errors.