terminal/idev: avoid magic numbers

Use XKB_CONTEXT_NO_FLAGS instead of magic 0.
terminal/idev: forward xkb-messages

Properly forward all XKB messages. You can use XKB_LOG_VERBOSITY= to
control the amount of messages sent by XKB. We explicitly set
XKB_LOG_LEVEL to 7 you can use SYSTEMD_LOG_LEVEL to control the log-level
generically.
localed: verify xkb keymaps after pk-queries

Make sure not to run xkb-keymap validation twice if pk-queries are used.
Move it below pk-checks.