Fix escaping of strings in UATs.

Fix escaping of strings in UATs.

Not only must characters that aren't printable ASCII characters be
escaped, backslashes must be escaped (as backslash is an escape
introducer) and double-quotes must be escaped (as double-quotes
encapsulate strings).

When constructing a string to hand to uat_load_str(), escape pathnames,
as they are likely to contain backslashes on Windows, could contain
backslashes on UN*X, and could contain quotes on UN*X and possibly
Windows.  (Arguably, we should escape all the string arguments

Bug: 11372
Change-Id: I594840327fa41895130903c3c612ba97d6c29df3
Reviewed-on: https://code.wireshark.org/review/9716
Reviewed-by: Guy Harris <guy@alum.mit.edu>