Make the parsing of command line getopt-compliant.

These commits make Why3 follow the getopt standard way of parsing command-line options.

Benefits:

• -Lfoo and -L foo both work.
• Short options can be concatenated, e.g., why3 wc -tfa.
• Arguments can be made optional, e.g., --smoke-detector instead of --smoke-detector=top.
• The getopt way is standard!

Downsides:

• Long options with arguments now need a = sign, e.g., --library foo is no longer recognized; --library=foo (or -L foo) should be used instead. Note that the --library=foo style was already recognized before, so compatibility can be ensured.