Convert illegal filename characters

[anonymous]

Convert filename characters that would be illegal on the destination platform to some other character.
For example other ftp packages rename the file, converting ":" (colon, in the source filename) to "_"
(underscore, in the destination filename) when transferring to DOS/Windows, which does not allow ":"
in a file's name.

Currently, when I use this package to transfer such a file, the destination filename is truncated at the
point of the invalid character and the file contains 0 bytes. The package claims the file was transferred
successfully, but it wasn't. So this is also a bug.

Thanks!

[Alexander Schuch]

Requested already:
[ 587170 ] Convert illegal filename characters