lib.ExecContext: Align .sudo() prototype to .run()

ExecContext's .sudo() omits many of run()'s parameters, and this commit adds them. To avoid redundancy around repeating and massaging the long parameter list of both functions and their return values, it also adds some deeper changes:
  - Make run(), _run(), sudo() and _sudo() always return instances of
    Result. Before it was allowed to return a triplet of stdout,
    stderr, and exit status.
  - Have ExecContext stay out of the business of decoding the result
    entirely. Result provides a convenience method .decode()
    operating on stdout and stderr and leaves the decision to the
    caller.
    This entails miniscule adaptations in calling code, namely in
    App.os_release, util.get_profile_env() and CmdListRepos._run().
  - Wrap the _run() and _sudo() callbacks in a context manager object
    of type CallContext to avoid code duplication.
  - Consistently name the first argument to run(), _run(), sudo() and
    _sudo() "cmd", not "args". The latter suggests that the caller is
    omitting the executable, which is not the case.
Signed-off-by: Jan Lindemann <jan@janware.com>

src/python/jw/pkg/App.py

@@ -301,7 +301,7 @@ class App(Base):
         if self.__os_release is None:
             result = self.call_async(self.exec_context.run(['/usr/bin/cat', '/etc/os-release'], throw=True))
             assert result.status == 0
-            self.__os_release = result.stdout.strip()
+            self.__os_release = result.decode().stdout.strip()
         return self.__os_release
 
     def os_release_field(self, key: str, throw: bool=False) -> str: