debootstrap: optionally exclude specific dependencies

debootstrap will always install the first alternative of a dependency. This setting can be used to exclude such a choice, but one has to be careful that the dependency is satisfied some other way.

This is intended to be used for the "usrmerge | usr-is-merged" dependency where debootstrap already installs "usr-is-merged" and should not pull in "usrmerge" as well.

Note that this patch changes the output of "debootstrap --print-debs" slightly: required packages are separated by space instead of newline as was already the case for base packages.