xkbcomp: handle XKB file include's better

The 'merge_mode' situation is quite messy, and we've introduced a
regression compared to original xkbcomp: when handling a composite
include statement, such as
    replace "foo(bar)+baz(bla)|doo:dee"
and merging the entire resulting *Info back into the including *Info,
we actually use the merge mode that is set by the last part (here it is
"augment" because of the '|'), when we should be using the one set for
the whole statement (here "replace").

We also take the opportunity to clean up a bit.

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