cmdline: Restore support for arbitrary global option handling.

The recent port to Getopt::Long broke the ability to use --foo=bar type
of options (where <foo> is a global option name known to the build
context). It also replaced the --module,foo=bar options with
--set-module-option-value=module,foo,bar.

While it's possible to use the latter syntax for global options (by
using 'global' as the module name) that is undocumented and may not work
forever. And either way, --foo options did use to work, and I noted it
was broken in the refactor.

So now that someone else has noticed I've reimplemented the feature.
It's difficult to do entirely from within Getopt::Long since the
documentation for that module gives you essentially one catch-all, which
only supports *non-options*.

So what I've done instead is to have kdesrc-build dynamically introspect
its list of global options and flags and then add them to the list of
valid options we pass to Getopt::Long. This also should mean that we
don't need to pass 'pass_through' as an option to Getopt::Long anymore,
though we'll see.

This code path won't be used for options that already have a command
line option (such as --async/--no-async) so that is still a misfeature
compared to how it was before, but I guess it can't be perfect. =D

I've tried to ensure that "flags" support the 'false' value to mean
boolean false, but be careful to pass false or 0 if that's what you
mean... specifying --dont-foo without giving it a value defaults it to
true.