The internal array of default key bindings is removed. Include the
file etc/input.conf at compile time (using the file2header tool), and
parse the default binds from etc/input.conf at startup time.
This lowers maintainance overhead, and makes sure the default bindings
and etc/input.conf don't deviate. Commit f30bf73bf2 already
made sure etc/input.conf matches the default bindings, so this commit
shouldn't change anything user-visible.