Sanitise bad characters in log file names.

Sanitise bad characters in log file names.

On Windows, colons are illegal in filenames, because they're part of
the path syntax. But colons can appear in automatically constructed
log file names, if an IPv6 address is expanded from the &H placeholder.

Now we coerce any such illegal characters to '.', which is a bit of a
bodge but should at least cause a log file to be generated.

(cherry picked from commit 64ec5e03d5362ed036e9de1a765085c571eaa3b7)