[darwinbuild] additive and subtractive dependencies

[Kevin Van Vechten]

I've modified the dependencies logic to enable additive and  
subtractive dependencies.  This allows a project to inherit all  
dependencies from a previous build, but add or remove a few entries.   
This also allows dependencies that are not detected by darwintrace to  
be manually added in a way that won't be over-written by future  
darwintrace invocations.

The syntax is as follows:

8C46:
     xnu = {
         dependencies = {
             build = (
                 adv_cmds,
                 awk,
                 bash,
                 bootstrap_cmds,
                 cctools,
                 ...

8C46od1:
     inherits = 8C46;
     xnu = {
         dependencies = {
             "+build" = (
                 tcsh,
             );
             "-build" = (
                 cctools,
             );

Would result in the following query from 8C46od1:

$ darwinxref dependencies -build xnu
adv_cmds
awk
bash
boostrap_cmds
tcsh
...