recognize blockers in .depend files

This adds very basic support for "!<module>" entries in dependency files,
which is supposed to ensure that certain modules don't get combined.
handle blockers from .depend files

make_dependency_table() detects if blocked modules are part of the dependency
table and, if so, raises an Exception (+ prints details to stderr).
fix dep blocker handling
control how --depfile blockers are handled

This allows altered/extended semantics for blocker entries in top-level
dependency files (--depfile <file>) using the following "actions":

* error/err/fail -- let make_dependency_table() fail if blocked modules are
                    required. (Identical to the action taken for non top-level
                    .depend files)
* warn           -- like error, but prints the blockers to stderr
* ignore         -- verbosely drop blockers
* exclude        -- verbosely add blockers to the exclude list (default action!)
* exclude-quiet  -- quietly add blockers to the exclude list
shlibcc 0.0.10