Guard against evil chars in bridge names

An evil/buggy application may create bridges or other network devices
with characters that may be misunderstood by the shell. Any application
that uses system(3) must take great care of issues like that.

Change-Id: I0caebe9a0423c6a04ba79bcaf5112ad267e8e377
Signed-off-by: Dan Kenigsberg <danken@redhat.com>