Refactor: fencing: convert fence_legacy to python

This eliminates the build dependency on perl, since the man page builder can
run fence_legacy -h without perl. (This was the last Perl script in the code
base, though there is still some embedded Perl in crm_report.)

The new argument parser works a bit differently (and more intuitively) than
the original one: fence_legacy arguments are now case-sensitive (e.g.
"--VeRSioN" is not allowed), and long options cannot be specified with a single
dash (e.g. use "--version" not "-version"). This should not cause any problems,
because fence_legacy should only be invoked by Pacemaker itself.