The Compose format has a feature which allows specifying certain
modifiers must (or must not) be present with a given keysym in the
sequence.

The grammar in imLcPrs.c and the Compose man page both do not match what
the code actually does (see the handling of the variables
`modifier_mask` and `modifier` in parseline() in imLcPrs.c, which are
eventually matched as `ev->state & modifier_mask == modifier`).

Also explicitly list the accepted modifier names, since they are
not standard (e.g. "Ctrl" instead of "Control").
Signed-off-by: Ran Benita <ran234@gmail.com>
Signed-off-by: James Cloos <cloos@jhcloos.com>
Backported-to-NX-by: Ulrich Sibiller <uli42@gmx.de>