ui/keymaps: Fix handling of erroneous include files

While errors in the keyboard layout named with -k are fatal, errors in
included files are reported, but otherwise ignored:

    $ cat worst
    include bad
    include worse
    $ ls -l bad worse
    ls: cannot access 'bad': No such file or directory
    ls: cannot access 'worse': No such file or directory
    $ qemu-system-x86_64 -nodefaults -S -monitor stdio -display vnc=:0 -k bad
    QEMU 3.0.50 monitor - type 'help' for more information
    (qemu) Could not read keymap file: 'bad'
    $ qemu-system-x86_64 -nodefaults -S -monitor stdio -display vnc=:0 -k worst
    QEMU 3.0.50 monitor - type 'help' for more information
    (qemu) Could not read keymap file: 'bad'
    Could not read keymap file: 'worse'

Fix that.

Note that parse_keyboard_layout() allocates the keymap, except when
it's parsing an include file.  To keep error handling simple, move the
memory management to its caller init_keyboard_layout().

Cc: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20181017082702.5581-26-armbru@redhat.com>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>