Make the context available to xkb_intern_atom

Make the context available to xkb_intern_atom

In preparation of contextualizing the atom table.
Since we touch every function call, also rename the function to
xkb_atom_intern, to match better with the rest (which will also be
renamed).

Signed-off-by: Ran Benita <ran234@gmail.com>
[daniels: Fixed for 'xkb' -> 'keymap'.]