bus-util: replace non-printable values with [unprintable]

bus-util: replace non-printable values with [unprintable]

Like I said in the previous commit, such values do not seem to appear in normal
use, but it's pretty hard to prove that all paths to assign values properly
check that they contain no spaces. So just in case some slip through, replace
values with spaces (in case of single-valued properties) or spaces and newlines
(in case of array proprties) with "[unprintable]". We were already doing it
in case of properties which we didn't know how to print, so this fits in well.
The advantage is the previous code which used escaping that a) this is easier
to spot, b) does not mess up printing of properties which were properly escaped
already.

v2:
- add comments