When generating the object-path from the object-id, instead of
using a blacklist of forbidden chars, use a whitelist of the
allowed object-path chars. Non-valid chars are replaced by '_'.
This assures we always have a valid, albeit maybe ugly, object-
path. Extend the test to include a object id with invalid chars.

python3 script to generate the #define from the freedesktop.org
specification:
import itertools

def main():
    """
    Valid Object Paths:
    [...]
    Each element must only contain the ASCII characters "[A-Z][a-z][0-9]_"
    [...]
      from https://dbus.freedesktop.org/doc/dbus-specification.html
    """

    name = "DBUS_OPATH_VALID_CHARS"
    chars = ''.join(
        itertools.chain(
            *[map(chr, range(ord(s), ord(e) + 1)) for s, e in [
                ('A', 'Z'),
                ('a', 'z'),
                ('0', '9'),
                ('_', '_')]]))
    print('#define %s "%s"' % (name, chars))

if __name__ == '__main__':
    main()