uu: Remove explicit ASCII check

Lines before the "begin" line can contain any kind of data, e.g. UTF-8
or other characters. Behave just like uudecode and skip any data in
front of it, not just ASCII.

This does not affect the sanity check of uu or base64 encoded
characters, since they already have tighter checks anyway.

Signed-off-by: Tobias Stoeckmann <tobias@stoeckmann.org>