New style is allow(foo), warn(foo), deny(foo) and forbid(foo),
mirrored by -A foo, -W foo, -D foo and -F foo on command line.
These replace -W no-foo, -W foo, -W err-foo, respectively.
Forbid is new, and means "deny, and you can't override it".
Under this scheme when parsing foo.rc the parser will also look for
foo.rs to fill in the crate-level module, and when evaluating a
directory module directive it will look for a .rs file with the
same name as the directory.
