Add "mode" parameter to add_binary

Some times add_binary is used to add non-binaries. Most of the time it's unintentional, but since there's now a special behaviour for scripts that warns if the interpreter is missing, it may now be desirable to add scripts with add_binary instead of add_file.

The particular case that comes to mind is add_runscript:

add_file "$script" "/hooks/$hookname" 755

That could be replaced with add_binary to produce warnings if /usr/bin/ash is missing (i.e. if the base hook or something else is missing).

Since runtime hook are not stored executable, we need add_binary to implement support for a third parameter for mode.