atom: drop {xkb_,}atom_strdup

Even though in 112cccb18ad1bc877b3c4a87fa536ea085c761b5 I said it might
be useful, it's not. So remove it.

Signed-off-by: Ran Benita <ran234@gmail.com>
atom: allow interning empty string

Which is different than XKB_ATOM_NONE, as in "" != NULL.

Signed-off-by: Ran Benita <ran234@gmail.com>
atom: allow passing NULLs to find_node_pointer()

Signed-off-by: Ran Benita <ran234@gmail.com>
atom: tiny style fixes

Signed-off-by: Ran Benita <ran234@gmail.com>
test/atom: add test for atom table

Mostly a random test.

Signed-off-by: Ran Benita <ran234@gmail.com>
atom: don't malloc every node separately

Instead of having a darray of pointers to malloc'ed atom_node's, make it
a darray of atom_node's directly.

This makes the code a bit simpler, saves on some malloc's, and the
memory gain/loss even out.

Unfortunately, we are no longer Three Star Programmers ;(
http://c2.com/cgi/wiki?ThreeStarProgrammer

Signed-off-by: Ran Benita <ran234@gmail.com>