keymap: Don't erase previous assignments

Don't use "KEYBOARD_KEY_xx=!" assignments (i. e. only enabling force-release)
if more general matches already explicitly set a key code before, to not
override the previously set value.

https://launchpad.net/bugs/1218433
keymap: Fix Dell vendor names

Spaces are dropped from vendor and product names in DMI modaliases, so a match
like "svnDell Inc.:" will never happen. Also, some machines use "Dell", some
"Dell Inc", some "Dell Inc.", so just match on "Dell*" to avoid all these
traps.

https://launchpad.net/bugs/1218433
keymap: Don't use spaces in DMI modalias matches

Spaces get dropped from DMI modaliases. Replace them with '*' to make them
robust against future changes in space escaping.