Don't print non-printable characters in AX.25 addresses.

Add a new routine to wsutil to make this easy: printable_char_or_period().

Bug: 10439
Change-Id: I0eb2bb6bc0676a1035c3d845b5e20276fa04de60
Reviewed-on: https://code.wireshark.org/review/3981
Petri-Dish: Alexis La Goutte <alexis.lagoutte@gmail.com>
Reviewed-by: Alexis La Goutte <alexis.lagoutte@gmail.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Michael Mann <mmann78@netscape.net>