Support extra-verbose builds:

- The bootstrap crate currently passes -v to Cargo if itself invoked
with -vv.  But Cargo supports -vv (to show build script output), so make
bootstrap pass that if itself invoked with -vvv.  (More specifically,
pass N '-v's to Cargo if invoked with N+1 of them.)

- bootstrap.py currently tries to pass on up to two '-v's to cargo when
building bootstrap, but incorrectly ('-v' is marked as 'store_true', so
argparse stores either False or True, ignoring multiple '-v's).  Fix
this, allow passing any number of '-v's, and make it consistent with
bootstrap's invocation of Cargo (i.e. subtract one from the number of
'-v's).

- Also improve bootstrap.py's config.toml 'parsing' to support arbitrary
verbosity levels, + allow command line to override it.

src/bootstrap/lib.rs

@@ -605,10 +605,6 @@ impl Build {
         self.verbosity > 0
     }
 
-    pub fn is_very_verbose(&self) -> bool {
-        self.verbosity > 1
-    }
-
     /// Prints a message if this build is configured in verbose mode.
     fn verbose(&self, msg: &str) {
         if self.is_verbose() {